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perlsub(1)             Perl Programmers Reference Guide             perlsub(1)




NAME

       perlsub - Perl subroutines


SYNOPSIS

       To declare subroutines:

           sub NAME;                     # A "forward" declaration.
           sub NAME(PROTO);              #  ditto, but with prototypes
           sub NAME : ATTRS;             #  with attributes
           sub NAME(PROTO) : ATTRS;      #  with attributes and prototypes

           sub NAME BLOCK                # A declaration and a definition.
           sub NAME(PROTO) BLOCK         #  ditto, but with prototypes
           sub NAME SIG BLOCK            #  with signature
           sub NAME : ATTRS BLOCK        #  with attributes
           sub NAME(PROTO) : ATTRS BLOCK #  with prototypes and attributes
           sub NAME : ATTRS SIG BLOCK    #  with attributes and signature

       To define an anonymous subroutine at runtime:

           $subref = sub BLOCK;                 # no proto
           $subref = sub (PROTO) BLOCK;         # with proto
           $subref = sub SIG BLOCK;             # with signature
           $subref = sub : ATTRS BLOCK;         # with attributes
           $subref = sub (PROTO) : ATTRS BLOCK; # with proto and attributes
           $subref = sub : ATTRS SIG BLOCK;     # with attribs and signature

       To import subroutines:

           use MODULE qw(NAME1 NAME2 NAME3);

       To call subroutines:

           NAME(LIST);    # & is optional with parentheses.
           NAME LIST;     # Parentheses optional if predeclared/imported.
           &NAME(LIST);   # Circumvent prototypes.
           &NAME;         # Makes current @_ visible to called subroutine.


DESCRIPTION

       Like many languages, Perl provides for user-defined subroutines.  These
       may be located anywhere in the main program, loaded in from other files
       via the "do", "require", or "use" keywords, or generated on the fly
       using "eval" or anonymous subroutines.  You can even call a function
       indirectly using a variable containing its name or a CODE reference.

       The Perl model for function call and return values is simple: all
       functions are passed as parameters one single flat list of scalars, and
       all functions likewise return to their caller one single flat list of
       scalars.  Any arrays or hashes in these call and return lists will
       collapse, losing their identities--but you may always use pass-by-
       reference instead to avoid this.  Both call and return lists may
       contain as many or as few scalar elements as you'd like.  (Often a
       function without an explicit return statement is called a subroutine,
       but there's really no difference from Perl's perspective.)

       Any arguments passed in show up in the array @_.  (They may also show
       up in lexical variables introduced by a signature; see "Signatures"
       below.)  Therefore, if you called a function with two arguments, those
       would be stored in $_[0] and $_[1].  The array @_ is a local array, but
       its elements are aliases for the actual scalar parameters.  In
       particular, if an element $_[0] is updated, the corresponding argument
       is updated (or an error occurs if it is not updatable).  If an argument
       is an array or hash element which did not exist when the function was
       called, that element is created only when (and if) it is modified or a
       reference to it is taken.  (Some earlier versions of Perl created the
       element whether or not the element was assigned to.)  Assigning to the
       whole array @_ removes that aliasing, and does not update any
       arguments.

       A "return" statement may be used to exit a subroutine, optionally
       specifying the returned value, which will be evaluated in the
       appropriate context (list, scalar, or void) depending on the context of
       the subroutine call.  If you specify no return value, the subroutine
       returns an empty list in list context, the undefined value in scalar
       context, or nothing in void context.  If you return one or more
       aggregates (arrays and hashes), these will be flattened together into
       one large indistinguishable list.

       If no "return" is found and if the last statement is an expression, its
       value is returned. If the last statement is a loop control structure
       like a "foreach" or a "while", the returned value is unspecified. The
       empty sub returns the empty list.

       Aside from an experimental facility (see "Signatures" below), Perl does
       not have named formal parameters.  In practice all you do is assign to
       a "my()" list of these.  Variables that aren't declared to be private
       are global variables.  For gory details on creating private variables,
       see "Private Variables via my()" and "Temporary Values via local()".
       To create protected environments for a set of functions in a separate
       package (and probably a separate file), see "Packages" in perlmod.

       Example:

           sub max {
               my $max = shift(@_);
               foreach $foo (@_) {
                   $max = $foo if $max < $foo;
               }
               return $max;
           }
           $bestday = max($mon,$tue,$wed,$thu,$fri);

       Example:

           # get a line, combining continuation lines
           #  that start with whitespace

           sub get_line {
               $thisline = $lookahead;  # global variables!
               LINE: while (defined($lookahead = <STDIN>)) {
                   if ($lookahead =~ /^[ \t]/) {
                       $thisline .= $lookahead;
                   }
                   else {
                       last LINE;
                   }
               }
               return $thisline;
           }

           $lookahead = <STDIN>;       # get first line
           while (defined($line = get_line())) {
               ...
           }

       Assigning to a list of private variables to name your arguments:

           sub maybeset {
               my($key, $value) = @_;
               $Foo{$key} = $value unless $Foo{$key};
           }

       Because the assignment copies the values, this also has the effect of
       turning call-by-reference into call-by-value.  Otherwise a function is
       free to do in-place modifications of @_ and change its caller's values.

           upcase_in($v1, $v2);  # this changes $v1 and $v2
           sub upcase_in {
               for (@_) { tr/a-z/A-Z/ }
           }

       You aren't allowed to modify constants in this way, of course.  If an
       argument were actually literal and you tried to change it, you'd take a
       (presumably fatal) exception.   For example, this won't work:

           upcase_in("frederick");

       It would be much safer if the "upcase_in()" function were written to
       return a copy of its parameters instead of changing them in place:

           ($v3, $v4) = upcase($v1, $v2);  # this doesn't change $v1 and $v2
           sub upcase {
               return unless defined wantarray;  # void context, do nothing
               my @parms = @_;
               for (@parms) { tr/a-z/A-Z/ }
               return wantarray ? @parms : $parms[0];
           }

       Notice how this (unprototyped) function doesn't care whether it was
       passed real scalars or arrays.  Perl sees all arguments as one big,
       long, flat parameter list in @_.  This is one area where Perl's simple
       argument-passing style shines.  The "upcase()" function would work
       perfectly well without changing the "upcase()" definition even if we
       fed it things like this:

           @newlist   = upcase(@list1, @list2);
           @newlist   = upcase( split /:/, $var );

       Do not, however, be tempted to do this:

           (@a, @b)   = upcase(@list1, @list2);

       Like the flattened incoming parameter list, the return list is also
       flattened on return.  So all you have managed to do here is stored
       everything in @a and made @b empty.  See "Pass by Reference" for
       alternatives.

       A subroutine may be called using an explicit "&" prefix.  The "&" is
       optional in modern Perl, as are parentheses if the subroutine has been
       predeclared.  The "&" is not optional when just naming the subroutine,
       such as when it's used as an argument to defined() or undef().  Nor is
       it optional when you want to do an indirect subroutine call with a
       subroutine name or reference using the "&$subref()" or "&{$subref}()"
       constructs, although the "$subref->()" notation solves that problem.
       See perlref for more about all that.

       Subroutines may be called recursively.  If a subroutine is called using
       the "&" form, the argument list is optional, and if omitted, no @_
       array is set up for the subroutine: the @_ array at the time of the
       call is visible to subroutine instead.  This is an efficiency mechanism
       that new users may wish to avoid.

           &foo(1,2,3);        # pass three arguments
           foo(1,2,3);         # the same

           foo();              # pass a null list
           &foo();             # the same

           &foo;               # foo() get current args, like foo(@_) !!
           foo;                # like foo() IFF sub foo predeclared, else "foo"

       Not only does the "&" form make the argument list optional, it also
       disables any prototype checking on arguments you do provide.  This is
       partly for historical reasons, and partly for having a convenient way
       to cheat if you know what you're doing.  See "Prototypes" below.

       Since Perl 5.16.0, the "__SUB__" token is available under "use feature
       'current_sub'" and "use 5.16.0".  It will evaluate to a reference to
       the currently-running sub, which allows for recursive calls without
       knowing your subroutine's name.

           use 5.16.0;
           my $factorial = sub {
             my ($x) = @_;
             return 1 if $x == 1;
             return($x * __SUB__->( $x - 1 ) );
           };

       The behaviour of "__SUB__" within a regex code block (such as
       "/(?{...})/") is subject to change.

       Subroutines whose names are in all upper case are reserved to the Perl
       core, as are modules whose names are in all lower case.  A subroutine
       in all capitals is a loosely-held convention meaning it will be called
       indirectly by the run-time system itself, usually due to a triggered
       event.  Subroutines whose name start with a left parenthesis are also
       reserved the same way. The following is a list of some subroutines that
       currently do special, pre-defined things.

       documented later in this document
           "AUTOLOAD"

       documented in perlmod
           "CLONE", "CLONE_SKIP",

       documented in perlobj
           "DESTROY"

       documented in perltie
           "BINMODE", "CLEAR", "CLOSE", "DELETE", "DESTROY", "EOF", "EXISTS",
           "EXTEND", "FETCH", "FETCHSIZE", "FILENO", "FIRSTKEY", "GETC",
           "NEXTKEY", "OPEN", "POP", "PRINT", "PRINTF", "PUSH", "READ",
           "READLINE", "SCALAR", "SEEK", "SHIFT", "SPLICE", "STORE",
           "STORESIZE", "TELL", "TIEARRAY", "TIEHANDLE", "TIEHASH",
           "TIESCALAR", "UNSHIFT", "UNTIE", "WRITE"

       documented in PerlIO::via
           "BINMODE", "CLEARERR", "CLOSE", "EOF", "ERROR", "FDOPEN", "FILENO",
           "FILL", "FLUSH", "OPEN", "POPPED", "PUSHED", "READ", "SEEK",
           "SETLINEBUF", "SYSOPEN", "TELL", "UNREAD", "UTF8", "WRITE"

       documented in perlfunc
           "import" , "unimport" , "INC"

       documented in UNIVERSAL
           "VERSION"

       documented in perldebguts
           "DB::DB", "DB::sub", "DB::lsub", "DB::goto", "DB::postponed"

       undocumented, used internally by the overload feature
           any starting with "("

       The "BEGIN", "UNITCHECK", "CHECK", "INIT" and "END" subroutines are not
       so much subroutines as named special code blocks, of which you can have
       more than one in a package, and which you can not call explicitly.  See
       "BEGIN, UNITCHECK, CHECK, INIT and END" in perlmod

   Signatures
       WARNING: Subroutine signatures are experimental.  The feature may be
       modified or removed in future versions of Perl.

       Perl has an experimental facility to allow a subroutine's formal
       parameters to be introduced by special syntax, separate from the
       procedural code of the subroutine body.  The formal parameter list is
       known as a signature.  The facility must be enabled first by a
       pragmatic declaration, "use feature 'signatures'", and it will produce
       a warning unless the "experimental::signatures" warnings category is
       disabled.

       The signature is part of a subroutine's body.  Normally the body of a
       subroutine is simply a braced block of code.  When using a signature,
       the signature is a parenthesised list that goes immediately before the
       braced block.  The signature declares lexical variables that are in
       scope for the block.  When the subroutine is called, the signature
       takes control first.  It populates the signature variables from the
       list of arguments that were passed.  If the argument list doesn't meet
       the requirements of the signature, then it will throw an exception.
       When the signature processing is complete, control passes to the block.

       Positional parameters are handled by simply naming scalar variables in
       the signature.  For example,

           sub foo ($left, $right) {
               return $left + $right;
           }

       takes two positional parameters, which must be filled at runtime by two
       arguments.  By default the parameters are mandatory, and it is not
       permitted to pass more arguments than expected.  So the above is
       equivalent to

           sub foo {
               die "Too many arguments for subroutine" unless @_ <= 2;
               die "Too few arguments for subroutine" unless @_ >= 2;
               my $left = $_[0];
               my $right = $_[1];
               return $left + $right;
           }

       An argument can be ignored by omitting the main part of the name from a
       parameter declaration, leaving just a bare "$" sigil.  For example,

           sub foo ($first, $, $third) {
               return "first=$first, third=$third";
           }

       Although the ignored argument doesn't go into a variable, it is still
       mandatory for the caller to pass it.

       A positional parameter is made optional by giving a default value,
       separated from the parameter name by "=":

           sub foo ($left, $right = 0) {
               return $left + $right;
           }

       The above subroutine may be called with either one or two arguments.
       The default value expression is evaluated when the subroutine is
       called, so it may provide different default values for different calls.
       It is only evaluated if the argument was actually omitted from the
       call.  For example,

           my $auto_id = 0;
           sub foo ($thing, $id = $auto_id++) {
               print "$thing has ID $id";
           }

       automatically assigns distinct sequential IDs to things for which no ID
       was supplied by the caller.  A default value expression may also refer
       to parameters earlier in the signature, making the default for one
       parameter vary according to the earlier parameters.  For example,

           sub foo ($first_name, $surname, $nickname = $first_name) {
               print "$first_name $surname is known as \"$nickname\"";
           }

       An optional parameter can be nameless just like a mandatory parameter.
       For example,

           sub foo ($thing, $ = 1) {
               print $thing;
           }

       The parameter's default value will still be evaluated if the
       corresponding argument isn't supplied, even though the value won't be
       stored anywhere.  This is in case evaluating it has important side
       effects.  However, it will be evaluated in void context, so if it
       doesn't have side effects and is not trivial it will generate a warning
       if the "void" warning category is enabled.  If a nameless optional
       parameter's default value is not important, it may be omitted just as
       the parameter's name was:

           sub foo ($thing, $=) {
               print $thing;
           }

       Optional positional parameters must come after all mandatory positional
       parameters.  (If there are no mandatory positional parameters then an
       optional positional parameters can be the first thing in the
       signature.)  If there are multiple optional positional parameters and
       not enough arguments are supplied to fill them all, they will be filled
       from left to right.

       After positional parameters, additional arguments may be captured in a
       slurpy parameter.  The simplest form of this is just an array variable:

           sub foo ($filter, @inputs) {
               print $filter->($_) foreach @inputs;
           }

       With a slurpy parameter in the signature, there is no upper limit on
       how many arguments may be passed.  A slurpy array parameter may be
       nameless just like a positional parameter, in which case its only
       effect is to turn off the argument limit that would otherwise apply:

           sub foo ($thing, @) {
               print $thing;
           }

       A slurpy parameter may instead be a hash, in which case the arguments
       available to it are interpreted as alternating keys and values.  There
       must be as many keys as values: if there is an odd argument then an
       exception will be thrown.  Keys will be stringified, and if there are
       duplicates then the later instance takes precedence over the earlier,
       as with standard hash construction.

           sub foo ($filter, %inputs) {
               print $filter->($_, $inputs{$_}) foreach sort keys %inputs;
           }

       A slurpy hash parameter may be nameless just like other kinds of
       parameter.  It still insists that the number of arguments available to
       it be even, even though they're not being put into a variable.

           sub foo ($thing, %) {
               print $thing;
           }

       A slurpy parameter, either array or hash, must be the last thing in the
       signature.  It may follow mandatory and optional positional parameters;
       it may also be the only thing in the signature.  Slurpy parameters
       cannot have default values: if no arguments are supplied for them then
       you get an empty array or empty hash.

       A signature may be entirely empty, in which case all it does is check
       that the caller passed no arguments:

           sub foo () {
               return 123;
           }

       When using a signature, the arguments are still available in the
       special array variable @_, in addition to the lexical variables of the
       signature.  There is a difference between the two ways of accessing the
       arguments: @_ aliases the arguments, but the signature variables get
       copies of the arguments.  So writing to a signature variable only
       changes that variable, and has no effect on the caller's variables, but
       writing to an element of @_ modifies whatever the caller used to supply
       that argument.

       There is a potential syntactic ambiguity between signatures and
       prototypes (see "Prototypes"), because both start with an opening
       parenthesis and both can appear in some of the same places, such as
       just after the name in a subroutine declaration.  For historical
       reasons, when signatures are not enabled, any opening parenthesis in
       such a context will trigger very forgiving prototype parsing.  Most
       signatures will be interpreted as prototypes in those circumstances,
       but won't be valid prototypes.  (A valid prototype cannot contain any
       alphabetic character.)  This will lead to somewhat confusing error
       messages.

       To avoid ambiguity, when signatures are enabled the special syntax for
       prototypes is disabled.  There is no attempt to guess whether a
       parenthesised group was intended to be a prototype or a signature.  To
       give a subroutine a prototype under these circumstances, use a
       prototype attribute.  For example,

           sub foo :prototype($) { $_[0] }

       It is entirely possible for a subroutine to have both a prototype and a
       signature.  They do different jobs: the prototype affects compilation
       of calls to the subroutine, and the signature puts argument values into
       lexical variables at runtime.  You can therefore write

           sub foo :prototype($$) ($left, $right) {
               return $left + $right;
           }

       The prototype attribute, and any other attributes, must come before the
       signature.  The signature always immediately precedes the block of the
       subroutine's body.

   Private Variables via my()
       Synopsis:

           my $foo;            # declare $foo lexically local
           my (@wid, %get);    # declare list of variables local
           my $foo = "flurp";  # declare $foo lexical, and init it
           my @oof = @bar;     # declare @oof lexical, and init it
           my $x : Foo = $y;   # similar, with an attribute applied

       WARNING: The use of attribute lists on "my" declarations is still
       evolving.  The current semantics and interface are subject to change.
       See attributes and Attribute::Handlers.

       The "my" operator declares the listed variables to be lexically
       confined to the enclosing block, conditional ("if/unless/elsif/else"),
       loop ("for/foreach/while/until/continue"), subroutine, "eval", or
       "do/require/use"'d file.  If more than one value is listed, the list
       must be placed in parentheses.  All listed elements must be legal
       lvalues.  Only alphanumeric identifiers may be lexically
       scoped--magical built-ins like $/ must currently be "local"ized with
       "local" instead.

       Unlike dynamic variables created by the "local" operator, lexical
       variables declared with "my" are totally hidden from the outside world,
       including any called subroutines.  This is true if it's the same
       subroutine called from itself or elsewhere--every call gets its own
       copy.

       This doesn't mean that a "my" variable declared in a statically
       enclosing lexical scope would be invisible.  Only dynamic scopes are
       cut off.   For example, the "bumpx()" function below has access to the
       lexical $x variable because both the "my" and the "sub" occurred at the
       same scope, presumably file scope.

           my $x = 10;
           sub bumpx { $x++ }

       An "eval()", however, can see lexical variables of the scope it is
       being evaluated in, so long as the names aren't hidden by declarations
       within the "eval()" itself.  See perlref.

       The parameter list to my() may be assigned to if desired, which allows
       you to initialize your variables.  (If no initializer is given for a
       particular variable, it is created with the undefined value.)  Commonly
       this is used to name input parameters to a subroutine.  Examples:

           $arg = "fred";        # "global" variable
           $n = cube_root(27);
           print "$arg thinks the root is $n\n";
        fred thinks the root is 3

           sub cube_root {
               my $arg = shift;  # name doesn't matter
               $arg **= 1/3;
               return $arg;
           }

       The "my" is simply a modifier on something you might assign to.  So
       when you do assign to variables in its argument list, "my" doesn't
       change whether those variables are viewed as a scalar or an array.  So

           my ($foo) = <STDIN>;                # WRONG?
           my @FOO = <STDIN>;

       both supply a list context to the right-hand side, while

           my $foo = <STDIN>;

       supplies a scalar context.  But the following declares only one
       variable:

           my $foo, $bar = 1;                  # WRONG

       That has the same effect as

           my $foo;
           $bar = 1;

       The declared variable is not introduced (is not visible) until after
       the current statement.  Thus,

           my $x = $x;

       can be used to initialize a new $x with the value of the old $x, and
       the expression

           my $x = 123 and $x == 123

       is false unless the old $x happened to have the value 123.

       Lexical scopes of control structures are not bounded precisely by the
       braces that delimit their controlled blocks; control expressions are
       part of that scope, too.  Thus in the loop

           while (my $line = <>) {
               $line = lc $line;
           } continue {
               print $line;
           }

       the scope of $line extends from its declaration throughout the rest of
       the loop construct (including the "continue" clause), but not beyond
       it.  Similarly, in the conditional

           if ((my $answer = <STDIN>) =~ /^yes$/i) {
               user_agrees();
           } elsif ($answer =~ /^no$/i) {
               user_disagrees();
           } else {
               chomp $answer;
               die "'$answer' is neither 'yes' nor 'no'";
           }

       the scope of $answer extends from its declaration through the rest of
       that conditional, including any "elsif" and "else" clauses, but not
       beyond it.  See "Simple Statements" in perlsyn for information on the
       scope of variables in statements with modifiers.

       The "foreach" loop defaults to scoping its index variable dynamically
       in the manner of "local".  However, if the index variable is prefixed
       with the keyword "my", or if there is already a lexical by that name in
       scope, then a new lexical is created instead.  Thus in the loop

           for my $i (1, 2, 3) {
               some_function();
           }

       the scope of $i extends to the end of the loop, but not beyond it,
       rendering the value of $i inaccessible within "some_function()".

       Some users may wish to encourage the use of lexically scoped variables.
       As an aid to catching implicit uses to package variables, which are
       always global, if you say

           use strict 'vars';

       then any variable mentioned from there to the end of the enclosing
       block must either refer to a lexical variable, be predeclared via "our"
       or "use vars", or else must be fully qualified with the package name.
       A compilation error results otherwise.  An inner block may countermand
       this with "no strict 'vars'".

       A "my" has both a compile-time and a run-time effect.  At compile time,
       the compiler takes notice of it.  The principal usefulness of this is
       to quiet "use strict 'vars'", but it is also essential for generation
       of closures as detailed in perlref.  Actual initialization is delayed
       until run time, though, so it gets executed at the appropriate time,
       such as each time through a loop, for example.

       Variables declared with "my" are not part of any package and are
       therefore never fully qualified with the package name.  In particular,
       you're not allowed to try to make a package variable (or other global)
       lexical:

           my $pack::var;      # ERROR!  Illegal syntax

       In fact, a dynamic variable (also known as package or global variables)
       are still accessible using the fully qualified "::" notation even while
       a lexical of the same name is also visible:

           package main;
           local $x = 10;
           my    $x = 20;
           print "$x and $::x\n";

       That will print out 20 and 10.

       You may declare "my" variables at the outermost scope of a file to hide
       any such identifiers from the world outside that file.  This is similar
       in spirit to C's static variables when they are used at the file level.
       To do this with a subroutine requires the use of a closure (an
       anonymous function that accesses enclosing lexicals).  If you want to
       create a private subroutine that cannot be called from outside that
       block, it can declare a lexical variable containing an anonymous sub
       reference:

           my $secret_version = '1.001-beta';
           my $secret_sub = sub { print $secret_version };
           &$secret_sub();

       As long as the reference is never returned by any function within the
       module, no outside module can see the subroutine, because its name is
       not in any package's symbol table.  Remember that it's not REALLY
       called $some_pack::secret_version or anything; it's just
       $secret_version, unqualified and unqualifiable.

       This does not work with object methods, however; all object methods
       have to be in the symbol table of some package to be found.  See
       "Function Templates" in perlref for something of a work-around to this.

   Persistent Private Variables
       There are two ways to build persistent private variables in Perl 5.10.
       First, you can simply use the "state" feature. Or, you can use
       closures, if you want to stay compatible with releases older than 5.10.

       Persistent variables via state()

       Beginning with Perl 5.10.0, you can declare variables with the "state"
       keyword in place of "my".  For that to work, though, you must have
       enabled that feature beforehand, either by using the "feature" pragma,
       or by using "-E" on one-liners (see feature).  Beginning with Perl
       5.16, the "CORE::state" form does not require the "feature" pragma.

       The "state" keyword creates a lexical variable (following the same
       scoping rules as "my") that persists from one subroutine call to the
       next.  If a state variable resides inside an anonymous subroutine, then
       each copy of the subroutine has its own copy of the state variable.
       However, the value of the state variable will still persist between
       calls to the same copy of the anonymous subroutine.  (Don't forget that
       "sub { ... }" creates a new subroutine each time it is executed.)

       For example, the following code maintains a private counter,
       incremented each time the gimme_another() function is called:

           use feature 'state';
           sub gimme_another { state $x; return ++$x }

       And this example uses anonymous subroutines to create separate
       counters:

           use feature 'state';
           sub create_counter {
               return sub { state $x; return ++$x }
           }

       Also, since $x is lexical, it can't be reached or modified by any Perl
       code outside.

       When combined with variable declaration, simple scalar assignment to
       "state" variables (as in "state $x = 42") is executed only the first
       time.  When such statements are evaluated subsequent times, the
       assignment is ignored.  The behavior of this sort of assignment to non-
       scalar variables is undefined.

       Persistent variables with closures

       Just because a lexical variable is lexically (also called statically)
       scoped to its enclosing block, "eval", or "do" FILE, this doesn't mean
       that within a function it works like a C static.  It normally works
       more like a C auto, but with implicit garbage collection.

       Unlike local variables in C or C++, Perl's lexical variables don't
       necessarily get recycled just because their scope has exited.  If
       something more permanent is still aware of the lexical, it will stick
       around.  So long as something else references a lexical, that lexical
       won't be freed--which is as it should be.  You wouldn't want memory
       being free until you were done using it, or kept around once you were
       done.  Automatic garbage collection takes care of this for you.

       This means that you can pass back or save away references to lexical
       variables, whereas to return a pointer to a C auto is a grave error.
       It also gives us a way to simulate C's function statics.  Here's a
       mechanism for giving a function private variables with both lexical
       scoping and a static lifetime.  If you do want to create something like
       C's static variables, just enclose the whole function in an extra
       block, and put the static variable outside the function but in the
       block.

           {
               my $secret_val = 0;
               sub gimme_another {
                   return ++$secret_val;
               }
           }
           # $secret_val now becomes unreachable by the outside
           # world, but retains its value between calls to gimme_another

       If this function is being sourced in from a separate file via "require"
       or "use", then this is probably just fine.  If it's all in the main
       program, you'll need to arrange for the "my" to be executed early,
       either by putting the whole block above your main program, or more
       likely, placing merely a "BEGIN" code block around it to make sure it
       gets executed before your program starts to run:

           BEGIN {
               my $secret_val = 0;
               sub gimme_another {
                   return ++$secret_val;
               }
           }

       See "BEGIN, UNITCHECK, CHECK, INIT and END" in perlmod about the
       special triggered code blocks, "BEGIN", "UNITCHECK", "CHECK", "INIT"
       and "END".

       If declared at the outermost scope (the file scope), then lexicals work
       somewhat like C's file statics.  They are available to all functions in
       that same file declared below them, but are inaccessible from outside
       that file.  This strategy is sometimes used in modules to create
       private variables that the whole module can see.

   Temporary Values via local()
       WARNING: In general, you should be using "my" instead of "local",
       because it's faster and safer.  Exceptions to this include the global
       punctuation variables, global filehandles and formats, and direct
       manipulation of the Perl symbol table itself.  "local" is mostly used
       when the current value of a variable must be visible to called
       subroutines.

       Synopsis:

           # localization of values

           local $foo;                # make $foo dynamically local
           local (@wid, %get);        # make list of variables local
           local $foo = "flurp";      # make $foo dynamic, and init it
           local @oof = @bar;         # make @oof dynamic, and init it

           local $hash{key} = "val";  # sets a local value for this hash entry
           delete local $hash{key};   # delete this entry for the current block
           local ($cond ? $v1 : $v2); # several types of lvalues support
                                      # localization

           # localization of symbols

           local *FH;                 # localize $FH, @FH, %FH, &FH  ...
           local *merlyn = *randal;   # now $merlyn is really $randal, plus
                                      #     @merlyn is really @randal, etc
           local *merlyn = 'randal';  # SAME THING: promote 'randal' to *randal
           local *merlyn = \$randal;  # just alias $merlyn, not @merlyn etc

       A "local" modifies its listed variables to be "local" to the enclosing
       block, "eval", or "do FILE"--and to any subroutine called from within
       that block.  A "local" just gives temporary values to global (meaning
       package) variables.  It does not create a local variable.  This is
       known as dynamic scoping.  Lexical scoping is done with "my", which
       works more like C's auto declarations.

       Some types of lvalues can be localized as well: hash and array elements
       and slices, conditionals (provided that their result is always
       localizable), and symbolic references.  As for simple variables, this
       creates new, dynamically scoped values.

       If more than one variable or expression is given to "local", they must
       be placed in parentheses.  This operator works by saving the current
       values of those variables in its argument list on a hidden stack and
       restoring them upon exiting the block, subroutine, or eval.  This means
       that called subroutines can also reference the local variable, but not
       the global one.  The argument list may be assigned to if desired, which
       allows you to initialize your local variables.  (If no initializer is
       given for a particular variable, it is created with an undefined
       value.)

       Because "local" is a run-time operator, it gets executed each time
       through a loop.  Consequently, it's more efficient to localize your
       variables outside the loop.

       Grammatical note on local()

       A "local" is simply a modifier on an lvalue expression.  When you
       assign to a "local"ized variable, the "local" doesn't change whether
       its list is viewed as a scalar or an array.  So

           local($foo) = <STDIN>;
           local @FOO = <STDIN>;

       both supply a list context to the right-hand side, while

           local $foo = <STDIN>;

       supplies a scalar context.

       Localization of special variables

       If you localize a special variable, you'll be giving a new value to it,
       but its magic won't go away.  That means that all side-effects related
       to this magic still work with the localized value.

       This feature allows code like this to work :

           # Read the whole contents of FILE in $slurp
           { local $/ = undef; $slurp = <FILE>; }

       Note, however, that this restricts localization of some values ; for
       example, the following statement dies, as of perl 5.10.0, with an error
       Modification of a read-only value attempted, because the $1 variable is
       magical and read-only :

           local $1 = 2;

       One exception is the default scalar variable: starting with perl 5.14
       "local($_)" will always strip all magic from $_, to make it possible to
       safely reuse $_ in a subroutine.

       WARNING: Localization of tied arrays and hashes does not currently work
       as described.  This will be fixed in a future release of Perl; in the
       meantime, avoid code that relies on any particular behaviour of
       localising tied arrays or hashes (localising individual elements is
       still okay).  See "Localising Tied Arrays and Hashes Is Broken" in
       perl58delta for more details.

       Localization of globs

       The construct

           local *name;

       creates a whole new symbol table entry for the glob "name" in the
       current package.  That means that all variables in its glob slot
       ($name, @name, %name, &name, and the "name" filehandle) are dynamically
       reset.

       This implies, among other things, that any magic eventually carried by
       those variables is locally lost.  In other words, saying "local */"
       will not have any effect on the internal value of the input record
       separator.

       Localization of elements of composite types

       It's also worth taking a moment to explain what happens when you
       "local"ize a member of a composite type (i.e. an array or hash
       element).  In this case, the element is "local"ized by name. This means
       that when the scope of the "local()" ends, the saved value will be
       restored to the hash element whose key was named in the "local()", or
       the array element whose index was named in the "local()".  If that
       element was deleted while the "local()" was in effect (e.g. by a
       "delete()" from a hash or a "shift()" of an array), it will spring back
       into existence, possibly extending an array and filling in the skipped
       elements with "undef".  For instance, if you say

           %hash = ( 'This' => 'is', 'a' => 'test' );
           @ary  = ( 0..5 );
           {
                local($ary[5]) = 6;
                local($hash{'a'}) = 'drill';
                while (my $e = pop(@ary)) {
                    print "$e . . .\n";
                    last unless $e > 3;
                }
                if (@ary) {
                    $hash{'only a'} = 'test';
                    delete $hash{'a'};
                }
           }
           print join(' ', map { "$_ $hash{$_}" } sort keys %hash),".\n";
           print "The array has ",scalar(@ary)," elements: ",
                 join(', ', map { defined $_ ? $_ : 'undef' } @ary),"\n";

       Perl will print

           6 . . .
           4 . . .
           3 . . .
           This is a test only a test.
           The array has 6 elements: 0, 1, 2, undef, undef, 5

       The behavior of local() on non-existent members of composite types is
       subject to change in future.

       Localized deletion of elements of composite types

       You can use the "delete local $array[$idx]" and "delete local
       $hash{key}" constructs to delete a composite type entry for the current
       block and restore it when it ends. They return the array/hash value
       before the localization, which means that they are respectively
       equivalent to

           do {
               my $val = $array[$idx];
               local  $array[$idx];
               delete $array[$idx];
               $val
           }

       and

           do {
               my $val = $hash{key};
               local  $hash{key};
               delete $hash{key};
               $val
           }

       except that for those the "local" is scoped to the "do" block. Slices
       are also accepted.

           my %hash = (
            a => [ 7, 8, 9 ],
            b => 1,
           )

           {
            my $a = delete local $hash{a};
            # $a is [ 7, 8, 9 ]
            # %hash is (b => 1)

            {
             my @nums = delete local @$a[0, 2]
             # @nums is (7, 9)
             # $a is [ undef, 8 ]

             $a[0] = 999; # will be erased when the scope ends
            }
            # $a is back to [ 7, 8, 9 ]

           }
           # %hash is back to its original state

   Lvalue subroutines
       It is possible to return a modifiable value from a subroutine.  To do
       this, you have to declare the subroutine to return an lvalue.

           my $val;
           sub canmod : lvalue {
               $val;  # or:  return $val;
           }
           sub nomod {
               $val;
           }

           canmod() = 5;   # assigns to $val
           nomod()  = 5;   # ERROR

       The scalar/list context for the subroutine and for the right-hand side
       of assignment is determined as if the subroutine call is replaced by a
       scalar. For example, consider:

           data(2,3) = get_data(3,4);

       Both subroutines here are called in a scalar context, while in:

           (data(2,3)) = get_data(3,4);

       and in:

           (data(2),data(3)) = get_data(3,4);

       all the subroutines are called in a list context.

       Lvalue subroutines are convenient, but you have to keep in mind that,
       when used with objects, they may violate encapsulation. A normal
       mutator can check the supplied argument before setting the attribute it
       is protecting, an lvalue subroutine cannot. If you require any special
       processing when storing and retrieving the values, consider using the
       CPAN module Sentinel or something similar.

   Lexical Subroutines
       WARNING: Lexical subroutines are still experimental.  The feature may
       be modified or removed in future versions of Perl.

       Lexical subroutines are only available under the "use feature
       'lexical_subs'" pragma, which produces a warning unless the
       "experimental::lexical_subs" warnings category is disabled.

       Beginning with Perl 5.18, you can declare a private subroutine with
       "my" or "state".  As with state variables, the "state" keyword is only
       available under "use feature 'state'" or "use 5.010" or higher.

       These subroutines are only visible within the block in which they are
       declared, and only after that declaration:

           no warnings "experimental::lexical_subs";
           use feature 'lexical_subs';

           foo();              # calls the package/global subroutine
           state sub foo {
               foo();          # also calls the package subroutine
           }
           foo();              # calls "state" sub
           my $ref = \&foo;    # take a reference to "state" sub

           my sub bar { ... }
           bar();              # calls "my" sub

       To use a lexical subroutine from inside the subroutine itself, you must
       predeclare it.  The "sub foo {...}" subroutine definition syntax
       respects any previous "my sub;" or "state sub;" declaration.

           my sub baz;         # predeclaration
           sub baz {           # define the "my" sub
               baz();          # recursive call
           }

       "state sub" vs "my sub"

       What is the difference between "state" subs and "my" subs?  Each time
       that execution enters a block when "my" subs are declared, a new copy
       of each sub is created.  "State" subroutines persist from one execution
       of the containing block to the next.

       So, in general, "state" subroutines are faster.  But "my" subs are
       necessary if you want to create closures:

           no warnings "experimental::lexical_subs";
           use feature 'lexical_subs';

           sub whatever {
               my $x = shift;
               my sub inner {
                   ... do something with $x ...
               }
               inner();
           }

       In this example, a new $x is created when "whatever" is called, and
       also a new "inner", which can see the new $x.  A "state" sub will only
       see the $x from the first call to "whatever".

       "our" subroutines

       Like "our $variable", "our sub" creates a lexical alias to the package
       subroutine of the same name.

       The two main uses for this are to switch back to using the package sub
       inside an inner scope:

           no warnings "experimental::lexical_subs";
           use feature 'lexical_subs';

           sub foo { ... }

           sub bar {
               my sub foo { ... }
               {
                   # need to use the outer foo here
                   our sub foo;
                   foo();
               }
           }

       and to make a subroutine visible to other packages in the same scope:

           package MySneakyModule;

           no warnings "experimental::lexical_subs";
           use feature 'lexical_subs';

           our sub do_something { ... }

           sub do_something_with_caller {
               package DB;
               () = caller 1;          # sets @DB::args
               do_something(@args);    # uses MySneakyModule::do_something
           }

   Passing Symbol Table Entries (typeglobs)
       WARNING: The mechanism described in this section was originally the
       only way to simulate pass-by-reference in older versions of Perl.
       While it still works fine in modern versions, the new reference
       mechanism is generally easier to work with.  See below.

       Sometimes you don't want to pass the value of an array to a subroutine
       but rather the name of it, so that the subroutine can modify the global
       copy of it rather than working with a local copy.  In perl you can
       refer to all objects of a particular name by prefixing the name with a
       star: *foo.  This is often known as a "typeglob", because the star on
       the front can be thought of as a wildcard match for all the funny
       prefix characters on variables and subroutines and such.

       When evaluated, the typeglob produces a scalar value that represents
       all the objects of that name, including any filehandle, format, or
       subroutine.  When assigned to, it causes the name mentioned to refer to
       whatever "*" value was assigned to it.  Example:

           sub doubleary {
               local(*someary) = @_;
               foreach $elem (@someary) {
                   $elem *= 2;
               }
           }
           doubleary(*foo);
           doubleary(*bar);

       Scalars are already passed by reference, so you can modify scalar
       arguments without using this mechanism by referring explicitly to $_[0]
       etc.  You can modify all the elements of an array by passing all the
       elements as scalars, but you have to use the "*" mechanism (or the
       equivalent reference mechanism) to "push", "pop", or change the size of
       an array.  It will certainly be faster to pass the typeglob (or
       reference).

       Even if you don't want to modify an array, this mechanism is useful for
       passing multiple arrays in a single LIST, because normally the LIST
       mechanism will merge all the array values so that you can't extract out
       the individual arrays.  For more on typeglobs, see "Typeglobs and
       Filehandles" in perldata.

   When to Still Use local()
       Despite the existence of "my", there are still three places where the
       "local" operator still shines.  In fact, in these three places, you
       must use "local" instead of "my".

       1.  You need to give a global variable a temporary value, especially
           $_.

           The global variables, like @ARGV or the punctuation variables, must
           be "local"ized with "local()".  This block reads in /etc/motd, and
           splits it up into chunks separated by lines of equal signs, which
           are placed in @Fields.

               {
                   local @ARGV = ("/etc/motd");
                   local $/ = undef;
                   local $_ = <>;
                   @Fields = split /^\s*=+\s*$/;
               }

           It particular, it's important to "local"ize $_ in any routine that
           assigns to it.  Look out for implicit assignments in "while"
           conditionals.

       2.  You need to create a local file or directory handle or a local
           function.

           A function that needs a filehandle of its own must use "local()" on
           a complete typeglob.   This can be used to create new symbol table
           entries:

               sub ioqueue {
                   local  (*READER, *WRITER);    # not my!
                   pipe    (READER,  WRITER)     or die "pipe: $!";
                   return (*READER, *WRITER);
               }
               ($head, $tail) = ioqueue();

           See the Symbol module for a way to create anonymous symbol table
           entries.

           Because assignment of a reference to a typeglob creates an alias,
           this can be used to create what is effectively a local function, or
           at least, a local alias.

               {
                   local *grow = \&shrink; # only until this block exits
                   grow();                # really calls shrink()
                   move();                # if move() grow()s, it shrink()s too
               }
               grow();                    # get the real grow() again

           See "Function Templates" in perlref for more about manipulating
           functions by name in this way.

       3.  You want to temporarily change just one element of an array or
           hash.

           You can "local"ize just one element of an aggregate.  Usually this
           is done on dynamics:

               {
                   local $SIG{INT} = 'IGNORE';
                   funct();                            # uninterruptible
               }
               # interruptibility automatically restored here

           But it also works on lexically declared aggregates.

   Pass by Reference
       If you want to pass more than one array or hash into a function--or
       return them from it--and have them maintain their integrity, then
       you're going to have to use an explicit pass-by-reference.  Before you
       do that, you need to understand references as detailed in perlref.
       This section may not make much sense to you otherwise.

       Here are a few simple examples.  First, let's pass in several arrays to
       a function and have it "pop" all of then, returning a new list of all
       their former last elements:

           @tailings = popmany ( \@a, \@b, \@c, \@d );

           sub popmany {
               my $aref;
               my @retlist = ();
               foreach $aref ( @_ ) {
                   push @retlist, pop @$aref;
               }
               return @retlist;
           }

       Here's how you might write a function that returns a list of keys
       occurring in all the hashes passed to it:

           @common = inter( \%foo, \%bar, \%joe );
           sub inter {
               my ($k, $href, %seen); # locals
               foreach $href (@_) {
                   while ( $k = each %$href ) {
                       $seen{$k}++;
                   }
               }
               return grep { $seen{$_} == @_ } keys %seen;
           }

       So far, we're using just the normal list return mechanism.  What
       happens if you want to pass or return a hash?  Well, if you're using
       only one of them, or you don't mind them concatenating, then the normal
       calling convention is ok, although a little expensive.

       Where people get into trouble is here:

           (@a, @b) = func(@c, @d);
       or
           (%a, %b) = func(%c, %d);

       That syntax simply won't work.  It sets just @a or %a and clears the @b
       or %b.  Plus the function didn't get passed into two separate arrays or
       hashes: it got one long list in @_, as always.

       If you can arrange for everyone to deal with this through references,
       it's cleaner code, although not so nice to look at.  Here's a function
       that takes two array references as arguments, returning the two array
       elements in order of how many elements they have in them:

           ($aref, $bref) = func(\@c, \@d);
           print "@$aref has more than @$bref\n";
           sub func {
               my ($cref, $dref) = @_;
               if (@$cref > @$dref) {
                   return ($cref, $dref);
               } else {
                   return ($dref, $cref);
               }
           }

       It turns out that you can actually do this also:

           (*a, *b) = func(\@c, \@d);
           print "@a has more than @b\n";
           sub func {
               local (*c, *d) = @_;
               if (@c > @d) {
                   return (\@c, \@d);
               } else {
                   return (\@d, \@c);
               }
           }

       Here we're using the typeglobs to do symbol table aliasing.  It's a tad
       subtle, though, and also won't work if you're using "my" variables,
       because only globals (even in disguise as "local"s) are in the symbol
       table.

       If you're passing around filehandles, you could usually just use the
       bare typeglob, like *STDOUT, but typeglobs references work, too.  For
       example:

           splutter(\*STDOUT);
           sub splutter {
               my $fh = shift;
               print $fh "her um well a hmmm\n";
           }

           $rec = get_rec(\*STDIN);
           sub get_rec {
               my $fh = shift;
               return scalar <$fh>;
           }

       If you're planning on generating new filehandles, you could do this.
       Notice to pass back just the bare *FH, not its reference.

           sub openit {
               my $path = shift;
               local *FH;
               return open (FH, $path) ? *FH : undef;
           }

   Prototypes
       Perl supports a very limited kind of compile-time argument checking
       using function prototyping.  This can be declared in either the PROTO
       section or with a prototype attribute.  If you declare either of

           sub mypush (+@)
           sub mypush :prototype(+@)

       then "mypush()" takes arguments exactly like "push()" does.

       If subroutine signatures are enabled (see "Signatures"), then the
       shorter PROTO syntax is unavailable, because it would clash with
       signatures.  In that case, a prototype can only be declared in the form
       of an attribute.

       The function declaration must be visible at compile time.  The
       prototype affects only interpretation of new-style calls to the
       function, where new-style is defined as not using the "&" character.
       In other words, if you call it like a built-in function, then it
       behaves like a built-in function.  If you call it like an old-fashioned
       subroutine, then it behaves like an old-fashioned subroutine.  It
       naturally falls out from this rule that prototypes have no influence on
       subroutine references like "\&foo" or on indirect subroutine calls like
       "&{$subref}" or "$subref->()".

       Method calls are not influenced by prototypes either, because the
       function to be called is indeterminate at compile time, since the exact
       code called depends on inheritance.

       Because the intent of this feature is primarily to let you define
       subroutines that work like built-in functions, here are prototypes for
       some other functions that parse almost exactly like the corresponding
       built-in.

          Declared as             Called as

          sub mylink ($$)         mylink $old, $new
          sub myvec ($$$)         myvec $var, $offset, 1
          sub myindex ($$;$)      myindex &getstring, "substr"
          sub mysyswrite ($$$;$)  mysyswrite $buf, 0, length($buf) - $off, $off
          sub myreverse (@)       myreverse $a, $b, $c
          sub myjoin ($@)         myjoin ":", $a, $b, $c
          sub mypop (+)           mypop @array
          sub mysplice (+$$@)     mysplice @array, 0, 2, @pushme
          sub mykeys (+)          mykeys %{$hashref}
          sub myopen (*;$)        myopen HANDLE, $name
          sub mypipe (**)         mypipe READHANDLE, WRITEHANDLE
          sub mygrep (&@)         mygrep { /foo/ } $a, $b, $c
          sub myrand (;$)         myrand 42
          sub mytime ()           mytime

       Any backslashed prototype character represents an actual argument that
       must start with that character (optionally preceded by "my", "our" or
       "local"), with the exception of "$", which will accept any scalar
       lvalue expression, such as "$foo = 7" or "my_function()->[0]". The
       value passed as part of @_ will be a reference to the actual argument
       given in the subroutine call, obtained by applying "\" to that
       argument.

       You can use the "\[]" backslash group notation to specify more than one
       allowed argument type. For example:

           sub myref (\[$@%&*])

       will allow calling myref() as

           myref $var
           myref @array
           myref %hash
           myref &sub
           myref *glob

       and the first argument of myref() will be a reference to a scalar, an
       array, a hash, a code, or a glob.

       Unbackslashed prototype characters have special meanings.  Any
       unbackslashed "@" or "%" eats all remaining arguments, and forces list
       context.  An argument represented by "$" forces scalar context.  An "&"
       requires an anonymous subroutine, which, if passed as the first
       argument, does not require the "sub" keyword or a subsequent comma.

       A "*" allows the subroutine to accept a bareword, constant, scalar
       expression, typeglob, or a reference to a typeglob in that slot.  The
       value will be available to the subroutine either as a simple scalar, or
       (in the latter two cases) as a reference to the typeglob.  If you wish
       to always convert such arguments to a typeglob reference, use
       Symbol::qualify_to_ref() as follows:

           use Symbol 'qualify_to_ref';

           sub foo (*) {
               my $fh = qualify_to_ref(shift, caller);
               ...
           }

       The "+" prototype is a special alternative to "$" that will act like
       "\[@%]" when given a literal array or hash variable, but will otherwise
       force scalar context on the argument.  This is useful for functions
       which should accept either a literal array or an array reference as the
       argument:

           sub mypush (+@) {
               my $aref = shift;
               die "Not an array or arrayref" unless ref $aref eq 'ARRAY';
               push @$aref, @_;
           }

       When using the "+" prototype, your function must check that the
       argument is of an acceptable type.

       A semicolon (";") separates mandatory arguments from optional
       arguments.  It is redundant before "@" or "%", which gobble up
       everything else.

       As the last character of a prototype, or just before a semicolon, a "@"
       or a "%", you can use "_" in place of "$": if this argument is not
       provided, $_ will be used instead.

       Note how the last three examples in the table above are treated
       specially by the parser.  "mygrep()" is parsed as a true list operator,
       "myrand()" is parsed as a true unary operator with unary precedence the
       same as "rand()", and "mytime()" is truly without arguments, just like
       "time()".  That is, if you say

           mytime +2;

       you'll get "mytime() + 2", not mytime(2), which is how it would be
       parsed without a prototype.  If you want to force a unary function to
       have the same precedence as a list operator, add ";" to the end of the
       prototype:

           sub mygetprotobynumber($;);
           mygetprotobynumber $a > $b; # parsed as mygetprotobynumber($a > $b)

       The interesting thing about "&" is that you can generate new syntax
       with it, provided it's in the initial position:

           sub try (&@) {
               my($try,$catch) = @_;
               eval { &$try };
               if ($@) {
                   local $_ = $@;
                   &$catch;
               }
           }
           sub catch (&) { $_[0] }

           try {
               die "phooey";
           } catch {
               /phooey/ and print "unphooey\n";
           };

       That prints "unphooey".  (Yes, there are still unresolved issues having
       to do with visibility of @_.  I'm ignoring that question for the
       moment.  (But note that if we make @_ lexically scoped, those anonymous
       subroutines can act like closures... (Gee, is this sounding a little
       Lispish?  (Never mind.))))

       And here's a reimplementation of the Perl "grep" operator:

           sub mygrep (&@) {
               my $code = shift;
               my @result;
               foreach $_ (@_) {
                   push(@result, $_) if &$code;
               }
               @result;
           }

       Some folks would prefer full alphanumeric prototypes.  Alphanumerics
       have been intentionally left out of prototypes for the express purpose
       of someday in the future adding named, formal parameters.  The current
       mechanism's main goal is to let module writers provide better
       diagnostics for module users.  Larry feels the notation quite
       understandable to Perl programmers, and that it will not intrude
       greatly upon the meat of the module, nor make it harder to read.  The
       line noise is visually encapsulated into a small pill that's easy to
       swallow.

       If you try to use an alphanumeric sequence in a prototype you will
       generate an optional warning - "Illegal character in prototype...".
       Unfortunately earlier versions of Perl allowed the prototype to be used
       as long as its prefix was a valid prototype.  The warning may be
       upgraded to a fatal error in a future version of Perl once the majority
       of offending code is fixed.

       It's probably best to prototype new functions, not retrofit prototyping
       into older ones.  That's because you must be especially careful about
       silent impositions of differing list versus scalar contexts.  For
       example, if you decide that a function should take just one parameter,
       like this:

           sub func ($) {
               my $n = shift;
               print "you gave me $n\n";
           }

       and someone has been calling it with an array or expression returning a
       list:

           func(@foo);
           func( split /:/ );

       Then you've just supplied an automatic "scalar" in front of their
       argument, which can be more than a bit surprising.  The old @foo which
       used to hold one thing doesn't get passed in.  Instead, "func()" now
       gets passed in a 1; that is, the number of elements in @foo.  And the
       "split" gets called in scalar context so it starts scribbling on your
       @_ parameter list.  Ouch!

       If a sub has both a PROTO and a BLOCK, the prototype is not applied
       until after the BLOCK is completely defined.  This means that a
       recursive function with a prototype has to be predeclared for the
       prototype to take effect, like so:

               sub foo($$);
               sub foo($$) {
                       foo 1, 2;
               }

       This is all very powerful, of course, and should be used only in
       moderation to make the world a better place.

   Constant Functions
       Functions with a prototype of "()" are potential candidates for
       inlining.  If the result after optimization and constant folding is
       either a constant or a lexically-scoped scalar which has no other
       references, then it will be used in place of function calls made
       without "&".  Calls made using "&" are never inlined.  (See constant.pm
       for an easy way to declare most constants.)

       The following functions would all be inlined:

           sub pi ()           { 3.14159 }             # Not exact, but close.
           sub PI ()           { 4 * atan2 1, 1 }      # As good as it gets,
                                                       # and it's inlined, too!
           sub ST_DEV ()       { 0 }
           sub ST_INO ()       { 1 }

           sub FLAG_FOO ()     { 1 << 8 }
           sub FLAG_BAR ()     { 1 << 9 }
           sub FLAG_MASK ()    { FLAG_FOO | FLAG_BAR }

           sub OPT_BAZ ()      { not (0x1B58 & FLAG_MASK) }

           sub N () { int(OPT_BAZ) / 3 }

           sub FOO_SET () { 1 if FLAG_MASK & FLAG_FOO }

       Be aware that these will not be inlined; as they contain inner scopes,
       the constant folding doesn't reduce them to a single constant:

           sub foo_set () { if (FLAG_MASK & FLAG_FOO) { 1 } }

           sub baz_val () {
               if (OPT_BAZ) {
                   return 23;
               }
               else {
                   return 42;
               }
           }

       As alluded to earlier you can also declare inlined subs dynamically at
       BEGIN time if their body consists of a lexically-scoped scalar which
       has no other references. Only the first example here will be inlined:

           BEGIN {
               my $var = 1;
               no strict 'refs';
               *INLINED = sub () { $var };
           }

           BEGIN {
               my $var = 1;
               my $ref = \$var;
               no strict 'refs';
               *NOT_INLINED = sub () { $var };
           }

       A not so obvious caveat with this (see [RT #79908]) is that the
       variable will be immediately inlined, and will stop behaving like a
       normal lexical variable, e.g. this will print 79907, not 79908:

           BEGIN {
               my $x = 79907;
               *RT_79908 = sub () { $x };
               $x++;
           }
           print RT_79908(); # prints 79907

       If you really want a subroutine with a "()" prototype that returns a
       lexical variable you can easily force it to not be inlined by adding an
       explicit "return":

           BEGIN {
               my $x = 79907;
               *RT_79908 = sub () { return $x };
               $x++;
           }
           print RT_79908(); # prints 79908

       The easiest way to tell if a subroutine was inlined is by using
       B::Deparse, consider this example of two subroutines returning 1, one
       with a "()" prototype causing it to be inlined, and one without (with
       deparse output truncated for clarity):

        $ perl -MO=Deparse -le 'sub ONE { 1 } if (ONE) { print ONE if ONE }'
        sub ONE {
            1;
        }
        if (ONE ) {
            print ONE() if ONE ;
        }
        $ perl -MO=Deparse -le 'sub ONE () { 1 } if (ONE) { print ONE if ONE }'
        sub ONE () { 1 }
        do {
            print 1
        };

       If you redefine a subroutine that was eligible for inlining, you'll get
       a warning by default. You can use this warning to tell whether or not a
       particular subroutine is considered inlinable, since it's different
       than the warning for overriding non-inlined subroutines:

           $ perl -e 'sub one () {1} sub one () {2}'
           Constant subroutine one redefined at -e line 1.
           $ perl -we 'sub one {1} sub one {2}'
           Subroutine one redefined at -e line 1.

       The warning is considered severe enough not to be affected by the -w
       switch (or its absence) because previously compiled invocations of the
       function will still be using the old value of the function.  If you
       need to be able to redefine the subroutine, you need to ensure that it
       isn't inlined, either by dropping the "()" prototype (which changes
       calling semantics, so beware) or by thwarting the inlining mechanism in
       some other way, e.g. by adding an explicit "return":

           sub not_inlined () { return 23 }

   Overriding Built-in Functions
       Many built-in functions may be overridden, though this should be tried
       only occasionally and for good reason.  Typically this might be done by
       a package attempting to emulate missing built-in functionality on a
       non-Unix system.

       Overriding may be done only by importing the name from a module at
       compile time--ordinary predeclaration isn't good enough.  However, the
       "use subs" pragma lets you, in effect, predeclare subs via the import
       syntax, and these names may then override built-in ones:

           use subs 'chdir', 'chroot', 'chmod', 'chown';
           chdir $somewhere;
           sub chdir { ... }

       To unambiguously refer to the built-in form, precede the built-in name
       with the special package qualifier "CORE::".  For example, saying
       "CORE::open()" always refers to the built-in "open()", even if the
       current package has imported some other subroutine called "&open()"
       from elsewhere.  Even though it looks like a regular function call, it
       isn't: the CORE:: prefix in that case is part of Perl's syntax, and
       works for any keyword, regardless of what is in the CORE package.
       Taking a reference to it, that is, "\&CORE::open", only works for some
       keywords.  See CORE.

       Library modules should not in general export built-in names like "open"
       or "chdir" as part of their default @EXPORT list, because these may
       sneak into someone else's namespace and change the semantics
       unexpectedly.  Instead, if the module adds that name to @EXPORT_OK,
       then it's possible for a user to import the name explicitly, but not
       implicitly.  That is, they could say

           use Module 'open';

       and it would import the "open" override.  But if they said

           use Module;

       they would get the default imports without overrides.

       The foregoing mechanism for overriding built-in is restricted, quite
       deliberately, to the package that requests the import.  There is a
       second method that is sometimes applicable when you wish to override a
       built-in everywhere, without regard to namespace boundaries.  This is
       achieved by importing a sub into the special namespace
       "CORE::GLOBAL::".  Here is an example that quite brazenly replaces the
       "glob" operator with something that understands regular expressions.

           package REGlob;
           require Exporter;
           @ISA = 'Exporter';
           @EXPORT_OK = 'glob';

           sub import {
               my $pkg = shift;
               return unless @_;
               my $sym = shift;
               my $where = ($sym =~ s/^GLOBAL_// ? 'CORE::GLOBAL' : caller(0));
               $pkg->export($where, $sym, @_);
           }

           sub glob {
               my $pat = shift;
               my @got;
               if (opendir my $d, '.') {
                   @got = grep /$pat/, readdir $d;
                   closedir $d;
               }
               return @got;
           }
           1;

       And here's how it could be (ab)used:

           #use REGlob 'GLOBAL_glob';      # override glob() in ALL namespaces
           package Foo;
           use REGlob 'glob';              # override glob() in Foo:: only
           print for <^[a-z_]+\.pm\$>;     # show all pragmatic modules

       The initial comment shows a contrived, even dangerous example.  By
       overriding "glob" globally, you would be forcing the new (and
       subversive) behavior for the "glob" operator for every namespace,
       without the complete cognizance or cooperation of the modules that own
       those namespaces.  Naturally, this should be done with extreme
       caution--if it must be done at all.

       The "REGlob" example above does not implement all the support needed to
       cleanly override perl's "glob" operator.  The built-in "glob" has
       different behaviors depending on whether it appears in a scalar or list
       context, but our "REGlob" doesn't.  Indeed, many perl built-in have
       such context sensitive behaviors, and these must be adequately
       supported by a properly written override.  For a fully functional
       example of overriding "glob", study the implementation of
       "File::DosGlob" in the standard library.

       When you override a built-in, your replacement should be consistent (if
       possible) with the built-in native syntax.  You can achieve this by
       using a suitable prototype.  To get the prototype of an overridable
       built-in, use the "prototype" function with an argument of
       "CORE::builtin_name" (see "prototype" in perlfunc).

       Note however that some built-ins can't have their syntax expressed by a
       prototype (such as "system" or "chomp").  If you override them you
       won't be able to fully mimic their original syntax.

       The built-ins "do", "require" and "glob" can also be overridden, but
       due to special magic, their original syntax is preserved, and you don't
       have to define a prototype for their replacements.  (You can't override
       the "do BLOCK" syntax, though).

       "require" has special additional dark magic: if you invoke your
       "require" replacement as "require Foo::Bar", it will actually receive
       the argument "Foo/Bar.pm" in @_.  See "require" in perlfunc.

       And, as you'll have noticed from the previous example, if you override
       "glob", the "<*>" glob operator is overridden as well.

       In a similar fashion, overriding the "readline" function also overrides
       the equivalent I/O operator "<FILEHANDLE>". Also, overriding "readpipe"
       also overrides the operators "``" and "qx//".

       Finally, some built-ins (e.g. "exists" or "grep") can't be overridden.

   Autoloading
       If you call a subroutine that is undefined, you would ordinarily get an
       immediate, fatal error complaining that the subroutine doesn't exist.
       (Likewise for subroutines being used as methods, when the method
       doesn't exist in any base class of the class's package.)  However, if
       an "AUTOLOAD" subroutine is defined in the package or packages used to
       locate the original subroutine, then that "AUTOLOAD" subroutine is
       called with the arguments that would have been passed to the original
       subroutine.  The fully qualified name of the original subroutine
       magically appears in the global $AUTOLOAD variable of the same package
       as the "AUTOLOAD" routine.  The name is not passed as an ordinary
       argument because, er, well, just because, that's why.  (As an
       exception, a method call to a nonexistent "import" or "unimport" method
       is just skipped instead.  Also, if the AUTOLOAD subroutine is an XSUB,
       there are other ways to retrieve the subroutine name.  See "Autoloading
       with XSUBs" in perlguts for details.)

       Many "AUTOLOAD" routines load in a definition for the requested
       subroutine using eval(), then execute that subroutine using a special
       form of goto() that erases the stack frame of the "AUTOLOAD" routine
       without a trace.  (See the source to the standard module documented in
       AutoLoader, for example.)  But an "AUTOLOAD" routine can also just
       emulate the routine and never define it.   For example, let's pretend
       that a function that wasn't defined should just invoke "system" with
       those arguments.  All you'd do is:

           sub AUTOLOAD {
               my $program = $AUTOLOAD;
               $program =~ s/.*:://;
               system($program, @_);
           }
           date();
           who('am', 'i');
           ls('-l');

       In fact, if you predeclare functions you want to call that way, you
       don't even need parentheses:

           use subs qw(date who ls);
           date;
           who "am", "i";
           ls '-l';

       A more complete example of this is the Shell module on CPAN, which can
       treat undefined subroutine calls as calls to external programs.

       Mechanisms are available to help modules writers split their modules
       into autoloadable files.  See the standard AutoLoader module described
       in AutoLoader and in AutoSplit, the standard SelfLoader modules in
       SelfLoader, and the document on adding C functions to Perl code in
       perlxs.

   Subroutine Attributes
       A subroutine declaration or definition may have a list of attributes
       associated with it.  If such an attribute list is present, it is broken
       up at space or colon boundaries and treated as though a "use
       attributes" had been seen.  See attributes for details about what
       attributes are currently supported.  Unlike the limitation with the
       obsolescent "use attrs", the "sub : ATTRLIST" syntax works to associate
       the attributes with a pre-declaration, and not just with a subroutine
       definition.

       The attributes must be valid as simple identifier names (without any
       punctuation other than the '_' character).  They may have a parameter
       list appended, which is only checked for whether its parentheses
       ('(',')') nest properly.

       Examples of valid syntax (even though the attributes are unknown):

           sub fnord (&\%) : switch(10,foo(7,3))  :  expensive;
           sub plugh () : Ugly('\(") :Bad;
           sub xyzzy : _5x5 { ... }

       Examples of invalid syntax:

           sub fnord : switch(10,foo(); # ()-string not balanced
           sub snoid : Ugly('(');        # ()-string not balanced
           sub xyzzy : 5x5;              # "5x5" not a valid identifier
           sub plugh : Y2::north;        # "Y2::north" not a simple identifier
           sub snurt : foo + bar;        # "+" not a colon or space

       The attribute list is passed as a list of constant strings to the code
       which associates them with the subroutine.  In particular, the second
       example of valid syntax above currently looks like this in terms of how
       it's parsed and invoked:

           use attributes __PACKAGE__, \&plugh, q[Ugly('\(")], 'Bad';

       For further details on attribute lists and their manipulation, see
       attributes and Attribute::Handlers.


SEE ALSO

       See "Function Templates" in perlref for more about references and
       closures.  See perlxs(1) if you'd like to learn about calling C
       subroutines from Perl.  See perlembed(1) if you'd like to learn about
       calling Perl subroutines from C.  See perlmod to learn about bundling
       up your functions in separate files.  See perlmodlib(1) to learn what
       library modules come standard on your system.  See perlootut(1) to learn
       how to make object method calls.



perl v5.20.0                      2014-05-26                        perlsub(1)

perl 5.20.0 - Generated Sat May 31 13:01:32 CDT 2014