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PERLSYN(1pm)           Perl Programmers Reference Guide           PERLSYN(1pm)




NAME

       perlsyn - Perl syntax


DESCRIPTION

       A Perl program consists of a sequence of declarations and statements
       which run from the top to the bottom.  Loops, subroutines, and other
       control structures allow you to jump around within the code.

       Perl is a free-form language: you can format and indent it however you
       like.  Whitespace serves mostly to separate tokens, unlike languages
       like Python where it is an important part of the syntax, or Fortran
       where it is immaterial.

       Many of Perl's syntactic elements are optional.  Rather than requiring
       you to put parentheses around every function call and declare every
       variable, you can often leave such explicit elements off and Perl will
       figure out what you meant.  This is known as Do What I Mean,
       abbreviated DWIM.  It allows programmers to be lazy and to code in a
       style with which they are comfortable.

       Perl borrows syntax and concepts from many languages: awk, sed, C,
       Bourne Shell, Smalltalk, Lisp and even English.  Other languages have
       borrowed syntax from Perl, particularly its regular expression
       extensions.  So if you have programmed in another language you will see
       familiar pieces in Perl.  They often work the same, but see perltrap
       for information about how they differ.

   Declarations
       The only things you need to declare in Perl are report formats and
       subroutines (and sometimes not even subroutines).  A scalar variable
       holds the undefined value ("undef") until it has been assigned a
       defined value, which is anything other than "undef".  When used as a
       number, "undef" is treated as 0; when used as a string, it is treated
       as the empty string, ""; and when used as a reference that isn't being
       assigned to, it is treated as an error.  If you enable warnings, you'll
       be notified of an uninitialized value whenever you treat "undef" as a
       string or a number.  Well, usually.  Boolean contexts, such as:

           if ($a) {}

       are exempt from warnings (because they care about truth rather than
       definedness).  Operators such as "++", "--", "+=", "-=", and ".=", that
       operate on undefined variables such as:

           undef $a;
           $a++;

       are also always exempt from such warnings.

       A declaration can be put anywhere a statement can, but has no effect on
       the execution of the primary sequence of statements: declarations all
       take effect at compile time.  All declarations are typically put at the
       beginning or the end of the script.  However, if you're using
       lexically-scoped private variables created with "my()", "state()", or
       "our()", you'll have to make sure your format or subroutine definition
       is within the same block scope as the my if you expect to be able to
       access those private variables.

       Declaring a subroutine allows a subroutine name to be used as if it
       were a list operator from that point forward in the program.  You can
       declare a subroutine without defining it by saying "sub name", thus:

           sub myname;
           $me = myname $0             or die "can't get myname";

       A bare declaration like that declares the function to be a list
       operator, not a unary operator, so you have to be careful to use
       parentheses (or "or" instead of "||".)  The "||" operator binds too
       tightly to use after list operators; it becomes part of the last
       element.  You can always use parentheses around the list operators
       arguments to turn the list operator back into something that behaves
       more like a function call.  Alternatively, you can use the prototype
       "($)" to turn the subroutine into a unary operator:

         sub myname ($);
         $me = myname $0             || die "can't get myname";

       That now parses as you'd expect, but you still ought to get in the
       habit of using parentheses in that situation.  For more on prototypes,
       see perlsub.

       Subroutines declarations can also be loaded up with the "require"
       statement or both loaded and imported into your namespace with a "use"
       statement.  See perlmod for details on this.

       A statement sequence may contain declarations of lexically-scoped
       variables, but apart from declaring a variable name, the declaration
       acts like an ordinary statement, and is elaborated within the sequence
       of statements as if it were an ordinary statement.  That means it
       actually has both compile-time and run-time effects.

   Comments
       Text from a "#" character until the end of the line is a comment, and
       is ignored.  Exceptions include "#" inside a string or regular
       expression.

   Simple Statements
       The only kind of simple statement is an expression evaluated for its
       side-effects.  Every simple statement must be terminated with a
       semicolon, unless it is the final statement in a block, in which case
       the semicolon is optional.  But put the semicolon in anyway if the
       block takes up more than one line, because you may eventually add
       another line.  Note that there are operators like "eval {}", "sub {}",
       and "do {}" that look like compound statements, but aren't--they're
       just TERMs in an expression--and thus need an explicit termination when
       used as the last item in a statement.

   Truth and Falsehood
       The number 0, the strings '0' and "", the empty list "()", and "undef"
       are all false in a boolean context.  All other values are true.
       Negation of a true value by "!" or "not" returns a special false value.
       When evaluated as a string it is treated as "", but as a number, it is
       treated as 0.  Most Perl operators that return true or false behave
       this way.

   Statement Modifiers
       Any simple statement may optionally be followed by a SINGLE modifier,
       just before the terminating semicolon (or block ending).  The possible
       modifiers are:

           if EXPR
           unless EXPR
           while EXPR
           until EXPR
           for LIST
           foreach LIST
           when EXPR

       The "EXPR" following the modifier is referred to as the "condition".
       Its truth or falsehood determines how the modifier will behave.

       "if" executes the statement once if and only if the condition is true.
       "unless" is the opposite, it executes the statement unless the
       condition is true (that is, if the condition is false).

           print "Basset hounds got long ears" if length $ear >= 10;
           go_outside() and play() unless $is_raining;

       The "for(each)" modifier is an iterator: it executes the statement once
       for each item in the LIST (with $_ aliased to each item in turn).

           print "Hello $_!\n" for qw(world Dolly nurse);

       "while" repeats the statement while the condition is true.  "until"
       does the opposite, it repeats the statement until the condition is true
       (or while the condition is false):

           # Both of these count from 0 to 10.
           print $i++ while $i <= 10;
           print $j++ until $j >  10;

       The "while" and "until" modifiers have the usual ""while" loop"
       semantics (conditional evaluated first), except when applied to a
       "do"-BLOCK (or to the Perl4 "do"-SUBROUTINE statement), in which case
       the block executes once before the conditional is evaluated.

       This is so that you can write loops like:

           do {
               $line = <STDIN>;
               ...
           } until !defined($line) || $line eq ".\n"

       See "do" in perlfunc.  Note also that the loop control statements
       described later will NOT work in this construct, because modifiers
       don't take loop labels.  Sorry.  You can always put another block
       inside of it (for "next"/"redo") or around it (for "last") to do that
       sort of thing.

       For "next" or "redo", just double the braces:

           do {{
               next if $x == $y;
               # do something here
           }} until $x++ > $z;

       For "last", you have to be more elaborate and put braces around it:

           {
               do {
                   last if $x == $y**2;
                   # do something here
               } while $x++ <= $z;
           }

       If you need both "next" and "last", you have to do both and also use a
       loop label:

           LOOP: {
               do {{
                   next if $x == $y;
                   last LOOP if $x == $y**2;
                   # do something here
               }} until $x++ > $z;
           }

       NOTE: The behaviour of a "my", "state", or "our" modified with a
       statement modifier conditional or loop construct (for example, "my $x
       if ...") is undefined.  The value of the "my" variable may be "undef",
       any previously assigned value, or possibly anything else.  Don't rely
       on it.  Future versions of perl might do something different from the
       version of perl you try it out on.  Here be dragons.

       The "when" modifier is an experimental feature that first appeared in
       Perl 5.14.  To use it, you should include a "use v5.14" declaration.
       (Technically, it requires only the "switch" feature, but that aspect of
       it was not available before 5.14.)  Operative only from within a
       "foreach" loop or a "given" block, it executes the statement only if
       the smartmatch "$_ ~~ EXPR" is true.  If the statement executes, it is
       followed by a "next" from inside a "foreach" and "break" from inside a
       "given".

       Under the current implementation, the "foreach" loop can be anywhere
       within the "when" modifier's dynamic scope, but must be within the
       "given" block's lexical scope.  This restriction may be relaxed in a
       future release.  See "Switch Statements" below.

   Compound Statements
       In Perl, a sequence of statements that defines a scope is called a
       block.  Sometimes a block is delimited by the file containing it (in
       the case of a required file, or the program as a whole), and sometimes
       a block is delimited by the extent of a string (in the case of an
       eval).

       But generally, a block is delimited by curly brackets, also known as
       braces.  We will call this syntactic construct a BLOCK.

       The following compound statements may be used to control flow:

           if (EXPR) BLOCK
           if (EXPR) BLOCK else BLOCK
           if (EXPR) BLOCK elsif (EXPR) BLOCK ...
           if (EXPR) BLOCK elsif (EXPR) BLOCK ... else BLOCK

           unless (EXPR) BLOCK
           unless (EXPR) BLOCK else BLOCK
           unless (EXPR) BLOCK elsif (EXPR) BLOCK ...
           unless (EXPR) BLOCK elsif (EXPR) BLOCK ... else BLOCK

           given (EXPR) BLOCK

           LABEL while (EXPR) BLOCK
           LABEL while (EXPR) BLOCK continue BLOCK

           LABEL until (EXPR) BLOCK
           LABEL until (EXPR) BLOCK continue BLOCK

           LABEL for (EXPR; EXPR; EXPR) BLOCK
           LABEL for VAR (LIST) BLOCK
           LABEL for VAR (LIST) BLOCK continue BLOCK

           LABEL foreach (EXPR; EXPR; EXPR) BLOCK
           LABEL foreach VAR (LIST) BLOCK
           LABEL foreach VAR (LIST) BLOCK continue BLOCK

           LABEL BLOCK
           LABEL BLOCK continue BLOCK

           PHASE BLOCK

       The experimental "given" statement is not automatically enabled; see
       "Switch Statements" below for how to do so, and the attendant caveats.

       Unlike in C and Pascal, in Perl these are all defined in terms of
       BLOCKs, not statements.  This means that the curly brackets are
       required--no dangling statements allowed.  If you want to write
       conditionals without curly brackets, there are several other ways to do
       it.  The following all do the same thing:

           if (!open(FOO)) { die "Can't open $FOO: $!" }
           die "Can't open $FOO: $!" unless open(FOO);
           open(FOO)  || die "Can't open $FOO: $!";
           open(FOO) ? () : die "Can't open $FOO: $!";
               # a bit exotic, that last one

       The "if" statement is straightforward.  Because BLOCKs are always
       bounded by curly brackets, there is never any ambiguity about which
       "if" an "else" goes with.  If you use "unless" in place of "if", the
       sense of the test is reversed.  Like "if", "unless" can be followed by
       "else".  "unless" can even be followed by one or more "elsif"
       statements, though you may want to think twice before using that
       particular language construct, as everyone reading your code will have
       to think at least twice before they can understand what's going on.

       The "while" statement executes the block as long as the expression is
       true.  The "until" statement executes the block as long as the
       expression is false.  The LABEL is optional, and if present, consists
       of an identifier followed by a colon.  The LABEL identifies the loop
       for the loop control statements "next", "last", and "redo".  If the
       LABEL is omitted, the loop control statement refers to the innermost
       enclosing loop.  This may include dynamically looking back your call-
       stack at run time to find the LABEL.  Such desperate behavior triggers
       a warning if you use the "use warnings" pragma or the -w flag.

       If there is a "continue" BLOCK, it is always executed just before the
       conditional is about to be evaluated again.  Thus it can be used to
       increment a loop variable, even when the loop has been continued via
       the "next" statement.

       When a block is preceding by a compilation phase keyword such as
       "BEGIN", "END", "INIT", "CHECK", or "UNITCHECK", then the block will
       run only during the corresponding phase of execution.  See perlmod for
       more details.

       Extension modules can also hook into the Perl parser to define new
       kinds of compound statements.  These are introduced by a keyword which
       the extension recognizes, and the syntax following the keyword is
       defined entirely by the extension.  If you are an implementor, see
       "PL_keyword_plugin" in perlapi for the mechanism.  If you are using
       such a module, see the module's documentation for details of the syntax
       that it defines.

   Loop Control
       The "next" command starts the next iteration of the loop:

           LINE: while (<STDIN>) {
               next LINE if /^#/;      # discard comments
               ...
           }

       The "last" command immediately exits the loop in question.  The
       "continue" block, if any, is not executed:

           LINE: while (<STDIN>) {
               last LINE if /^$/;      # exit when done with header
               ...
           }

       The "redo" command restarts the loop block without evaluating the
       conditional again.  The "continue" block, if any, is not executed.
       This command is normally used by programs that want to lie to
       themselves about what was just input.

       For example, when processing a file like /etc/termcap.  If your input
       lines might end in backslashes to indicate continuation, you want to
       skip ahead and get the next record.

           while (<>) {
               chomp;
               if (s/\\$//) {
                   $_ .= <>;
                   redo unless eof();
               }
               # now process $_
           }

       which is Perl shorthand for the more explicitly written version:

           LINE: while (defined($line = <ARGV>)) {
               chomp($line);
               if ($line =~ s/\\$//) {
                   $line .= <ARGV>;
                   redo LINE unless eof(); # not eof(ARGV)!
               }
               # now process $line
           }

       Note that if there were a "continue" block on the above code, it would
       get executed only on lines discarded by the regex (since redo skips the
       continue block).  A continue block is often used to reset line counters
       or "m?pat?" one-time matches:

           # inspired by :1,$g/fred/s//WILMA/
           while (<>) {
               m?(fred)?    && s//WILMA $1 WILMA/;
               m?(barney)?  && s//BETTY $1 BETTY/;
               m?(homer)?   && s//MARGE $1 MARGE/;
           } continue {
               print "$ARGV $.: $_";
               close ARGV  if eof;             # reset $.
               reset       if eof;             # reset ?pat?
           }

       If the word "while" is replaced by the word "until", the sense of the
       test is reversed, but the conditional is still tested before the first
       iteration.

       Loop control statements don't work in an "if" or "unless", since they
       aren't loops.  You can double the braces to make them such, though.

           if (/pattern/) {{
               last if /fred/;
               next if /barney/; # same effect as "last",
                                 # but doesn't document as well
               # do something here
           }}

       This is caused by the fact that a block by itself acts as a loop that
       executes once, see "Basic BLOCKs".

       The form "while/if BLOCK BLOCK", available in Perl 4, is no longer
       available.   Replace any occurrence of "if BLOCK" by "if (do BLOCK)".

   For Loops
       Perl's C-style "for" loop works like the corresponding "while" loop;
       that means that this:

           for ($i = 1; $i < 10; $i++) {
               ...
           }

       is the same as this:

           $i = 1;
           while ($i < 10) {
               ...
           } continue {
               $i++;
           }

       There is one minor difference: if variables are declared with "my" in
       the initialization section of the "for", the lexical scope of those
       variables is exactly the "for" loop (the body of the loop and the
       control sections).

       As a special case, if the test in the "for" loop (or the corresponding
       "while" loop) is empty, it is treated as true.  That is, both

           for (;;) {
               ...
           }

       and

           while () {
               ...
           }

       are treated as infinite loops.

       Besides the normal array index looping, "for" can lend itself to many
       other interesting applications.  Here's one that avoids the problem you
       get into if you explicitly test for end-of-file on an interactive file
       descriptor causing your program to appear to hang.

           $on_a_tty = -t STDIN && -t STDOUT;
           sub prompt { print "yes? " if $on_a_tty }
           for ( prompt(); <STDIN>; prompt() ) {
               # do something
           }

       Using "readline" (or the operator form, "<EXPR>") as the conditional of
       a "for" loop is shorthand for the following.  This behaviour is the
       same as a "while" loop conditional.

           for ( prompt(); defined( $_ = <STDIN> ); prompt() ) {
               # do something
           }

   Foreach Loops
       The "foreach" loop iterates over a normal list value and sets the
       scalar variable VAR to be each element of the list in turn.  If the
       variable is preceded with the keyword "my", then it is lexically
       scoped, and is therefore visible only within the loop.  Otherwise, the
       variable is implicitly local to the loop and regains its former value
       upon exiting the loop.  If the variable was previously declared with
       "my", it uses that variable instead of the global one, but it's still
       localized to the loop.  This implicit localization occurs only in a
       "foreach" loop.

       The "foreach" keyword is actually a synonym for the "for" keyword, so
       you can use either.  If VAR is omitted, $_ is set to each value.

       If any element of LIST is an lvalue, you can modify it by modifying VAR
       inside the loop.  Conversely, if any element of LIST is NOT an lvalue,
       any attempt to modify that element will fail.  In other words, the
       "foreach" loop index variable is an implicit alias for each item in the
       list that you're looping over.

       If any part of LIST is an array, "foreach" will get very confused if
       you add or remove elements within the loop body, for example with
       "splice".   So don't do that.

       "foreach" probably won't do what you expect if VAR is a tied or other
       special variable.   Don't do that either.

       As of Perl 5.22, there is an experimental variant of this loop that
       accepts a variable preceded by a backslash for VAR, in which case the
       items in the LIST must be references.  The backslashed variable will
       become an alias to each referenced item in the LIST, which must be of
       the correct type.  The variable needn't be a scalar in this case, and
       the backslash may be followed by "my".  To use this form, you must
       enable the "refaliasing" feature via "use feature".  (See feature.  See
       also "Assigning to References" in perlref.)

       Examples:

           for (@ary) { s/foo/bar/ }

           for my $elem (@elements) {
               $elem *= 2;
           }

           for $count (reverse(1..10), "BOOM") {
               print $count, "\n";
               sleep(1);
           }

           for (1..15) { print "Merry Christmas\n"; }

           foreach $item (split(/:[\\\n:]*/, $ENV{TERMCAP})) {
               print "Item: $item\n";
           }

           use feature "refaliasing";
           no warnings "experimental::refaliasing";
           foreach \my %hash (@array_of_hash_references) {
               # do something which each %hash
           }

       Here's how a C programmer might code up a particular algorithm in Perl:

           for (my $i = 0; $i < @ary1; $i++) {
               for (my $j = 0; $j < @ary2; $j++) {
                   if ($ary1[$i] > $ary2[$j]) {
                       last; # can't go to outer :-(
                   }
                   $ary1[$i] += $ary2[$j];
               }
               # this is where that last takes me
           }

       Whereas here's how a Perl programmer more comfortable with the idiom
       might do it:

           OUTER: for my $wid (@ary1) {
           INNER:   for my $jet (@ary2) {
                       next OUTER if $wid > $jet;
                       $wid += $jet;
                    }
                 }

       See how much easier this is?  It's cleaner, safer, and faster.  It's
       cleaner because it's less noisy.  It's safer because if code gets added
       between the inner and outer loops later on, the new code won't be
       accidentally executed.  The "next" explicitly iterates the other loop
       rather than merely terminating the inner one.  And it's faster because
       Perl executes a "foreach" statement more rapidly than it would the
       equivalent "for" loop.

       Perceptive Perl hackers may have noticed that a "for" loop has a return
       value, and that this value can be captured by wrapping the loop in a
       "do" block.  The reward for this discovery is this cautionary advice:
       The return value of a "for" loop is unspecified and may change without
       notice.  Do not rely on it.

   Basic BLOCKs
       A BLOCK by itself (labeled or not) is semantically equivalent to a loop
       that executes once.  Thus you can use any of the loop control
       statements in it to leave or restart the block.  (Note that this is NOT
       true in "eval{}", "sub{}", or contrary to popular belief "do{}" blocks,
       which do NOT count as loops.)  The "continue" block is optional.

       The BLOCK construct can be used to emulate case structures.

           SWITCH: {
               if (/^abc/) { $abc = 1; last SWITCH; }
               if (/^def/) { $def = 1; last SWITCH; }
               if (/^xyz/) { $xyz = 1; last SWITCH; }
               $nothing = 1;
           }

       You'll also find that "foreach" loop used to create a topicalizer and a
       switch:

           SWITCH:
           for ($var) {
               if (/^abc/) { $abc = 1; last SWITCH; }
               if (/^def/) { $def = 1; last SWITCH; }
               if (/^xyz/) { $xyz = 1; last SWITCH; }
               $nothing = 1;
           }

       Such constructs are quite frequently used, both because older versions
       of Perl had no official "switch" statement, and also because the new
       version described immediately below remains experimental and can
       sometimes be confusing.

   Switch Statements
       Starting from Perl 5.10.1 (well, 5.10.0, but it didn't work right), you
       can say

           use feature "switch";

       to enable an experimental switch feature.  This is loosely based on an
       old version of a Perl 6 proposal, but it no longer resembles the Perl 6
       construct.   You also get the switch feature whenever you declare that
       your code prefers to run under a version of Perl that is 5.10 or later.
       For example:

           use v5.14;

       Under the "switch" feature, Perl gains the experimental keywords
       "given", "when", "default", "continue", and "break".  Starting from
       Perl 5.16, one can prefix the switch keywords with "CORE::" to access
       the feature without a "use feature" statement.  The keywords "given"
       and "when" are analogous to "switch" and "case" in other languages --
       though "continue" is not -- so the code in the previous section could
       be rewritten as

           use v5.10.1;
           for ($var) {
               when (/^abc/) { $abc = 1 }
               when (/^def/) { $def = 1 }
               when (/^xyz/) { $xyz = 1 }
               default       { $nothing = 1 }
           }

       The "foreach" is the non-experimental way to set a topicalizer.  If you
       wish to use the highly experimental "given", that could be written like
       this:

           use v5.10.1;
           given ($var) {
               when (/^abc/) { $abc = 1 }
               when (/^def/) { $def = 1 }
               when (/^xyz/) { $xyz = 1 }
               default       { $nothing = 1 }
           }

       As of 5.14, that can also be written this way:

           use v5.14;
           for ($var) {
               $abc = 1 when /^abc/;
               $def = 1 when /^def/;
               $xyz = 1 when /^xyz/;
               default { $nothing = 1 }
           }

       Or if you don't care to play it safe, like this:

           use v5.14;
           given ($var) {
               $abc = 1 when /^abc/;
               $def = 1 when /^def/;
               $xyz = 1 when /^xyz/;
               default { $nothing = 1 }
           }

       The arguments to "given" and "when" are in scalar context, and "given"
       assigns the $_ variable its topic value.

       Exactly what the EXPR argument to "when" does is hard to describe
       precisely, but in general, it tries to guess what you want done.
       Sometimes it is interpreted as "$_ ~~ EXPR", and sometimes it is not.
       It also behaves differently when lexically enclosed by a "given" block
       than it does when dynamically enclosed by a "foreach" loop.  The rules
       are far too difficult to understand to be described here.  See
       "Experimental Details on given and when" later on.

       Due to an unfortunate bug in how "given" was implemented between Perl
       5.10 and 5.16, under those implementations the version of $_ governed
       by "given" is merely a lexically scoped copy of the original, not a
       dynamically scoped alias to the original, as it would be if it were a
       "foreach" or under both the original and the current Perl 6 language
       specification.  This bug was fixed in Perl 5.18 (and lexicalized $_
       itself was removed in Perl 5.24).

       If your code still needs to run on older versions, stick to "foreach"
       for your topicalizer and you will be less unhappy.

   Goto
       Although not for the faint of heart, Perl does support a "goto"
       statement.  There are three forms: "goto"-LABEL, "goto"-EXPR, and
       "goto"-&NAME.  A loop's LABEL is not actually a valid target for a
       "goto"; it's just the name of the loop.

       The "goto"-LABEL form finds the statement labeled with LABEL and
       resumes execution there.  It may not be used to go into any construct
       that requires initialization, such as a subroutine or a "foreach" loop.
       It also can't be used to go into a construct that is optimized away.
       It can be used to go almost anywhere else within the dynamic scope,
       including out of subroutines, but it's usually better to use some other
       construct such as "last" or "die".  The author of Perl has never felt
       the need to use this form of "goto" (in Perl, that is--C is another
       matter).

       The "goto"-EXPR form expects a label name, whose scope will be resolved
       dynamically.  This allows for computed "goto"s per FORTRAN, but isn't
       necessarily recommended if you're optimizing for maintainability:

           goto(("FOO", "BAR", "GLARCH")[$i]);

       The "goto"-&NAME form is highly magical, and substitutes a call to the
       named subroutine for the currently running subroutine.  This is used by
       "AUTOLOAD()" subroutines that wish to load another subroutine and then
       pretend that the other subroutine had been called in the first place
       (except that any modifications to @_ in the current subroutine are
       propagated to the other subroutine.)  After the "goto", not even
       "caller()" will be able to tell that this routine was called first.

       In almost all cases like this, it's usually a far, far better idea to
       use the structured control flow mechanisms of "next", "last", or "redo"
       instead of resorting to a "goto".  For certain applications, the catch
       and throw pair of "eval{}" and die() for exception processing can also
       be a prudent approach.

   The Ellipsis Statement
       Beginning in Perl 5.12, Perl accepts an ellipsis, ""..."", as a
       placeholder for code that you haven't implemented yet.  This form of
       ellipsis, the unimplemented statement, should not be confused with the
       binary flip-flop "..." operator.  One is a statement and the other an
       operator.  (Perl doesn't usually confuse them because usually Perl can
       tell whether it wants an operator or a statement, but see below for
       exceptions.)

       When Perl 5.12 or later encounters an ellipsis statement, it parses
       this without error, but if and when you should actually try to execute
       it, Perl throws an exception with the text "Unimplemented":

           use v5.12;
           sub unimplemented { ... }
           eval { unimplemented() };
           if ($@ =~ /^Unimplemented at /) {
               say "I found an ellipsis!";
           }

       You can only use the elliptical statement to stand in for a complete
       statement.  These examples of how the ellipsis works:

           use v5.12;
           { ... }
           sub foo { ... }
           ...;
           eval { ... };
           sub somemeth {
               my $self = shift;
               ...;
           }
           $x = do {
               my $n;
               ...;
               say "Hurrah!";
               $n;
           };

       The elliptical statement cannot stand in for an expression that is part
       of a larger statement, since the "..." is also the three-dot version of
       the flip-flop operator (see "Range Operators" in perlop).

       These examples of attempts to use an ellipsis are syntax errors:

           use v5.12;

           print ...;
           open(my $fh, ">", "/dev/passwd") or ...;
           if ($condition && ... ) { say "Howdy" };

       There are some cases where Perl can't immediately tell the difference
       between an expression and a statement.  For instance, the syntax for a
       block and an anonymous hash reference constructor look the same unless
       there's something in the braces to give Perl a hint.  The ellipsis is a
       syntax error if Perl doesn't guess that the "{ ... }" is a block.  In
       that case, it doesn't think the "..." is an ellipsis because it's
       expecting an expression instead of a statement:

           @transformed = map { ... } @input;    # syntax error

       Inside your block, you can use a ";" before the ellipsis to denote that
       the "{ ... }" is a block and not a hash reference constructor.  Now the
       ellipsis works:

           @transformed = map {; ... } @input;   # ';' disambiguates

       Note: Some folks colloquially refer to this bit of punctuation as a
       "yada-yada" or "triple-dot", but its true name is actually an ellipsis.

   PODs: Embedded Documentation
       Perl has a mechanism for intermixing documentation with source code.
       While it's expecting the beginning of a new statement, if the compiler
       encounters a line that begins with an equal sign and a word, like this

           =head1 Here There Be Pods!

       Then that text and all remaining text up through and including a line
       beginning with "=cut" will be ignored.  The format of the intervening
       text is described in perlpod.

       This allows you to intermix your source code and your documentation
       text freely, as in

           =item snazzle($)

           The snazzle() function will behave in the most spectacular
           form that you can possibly imagine, not even excepting
           cybernetic pyrotechnics.

           =cut back to the compiler, nuff of this pod stuff!

           sub snazzle($) {
               my $thingie = shift;
               .........
           }

       Note that pod translators should look at only paragraphs beginning with
       a pod directive (it makes parsing easier), whereas the compiler
       actually knows to look for pod escapes even in the middle of a
       paragraph.  This means that the following secret stuff will be ignored
       by both the compiler and the translators.

           $a=3;
           =secret stuff
            warn "Neither POD nor CODE!?"
           =cut back
           print "got $a\n";

       You probably shouldn't rely upon the "warn()" being podded out forever.
       Not all pod translators are well-behaved in this regard, and perhaps
       the compiler will become pickier.

       One may also use pod directives to quickly comment out a section of
       code.

   Plain Old Comments (Not!)
       Perl can process line directives, much like the C preprocessor.  Using
       this, one can control Perl's idea of filenames and line numbers in
       error or warning messages (especially for strings that are processed
       with "eval()").  The syntax for this mechanism is almost the same as
       for most C preprocessors: it matches the regular expression

           # example: '# line 42 "new_filename.plx"'
           /^\#   \s*
             line \s+ (\d+)   \s*
             (?:\s("?)([^"]+)\g2)? \s*
            $/x

       with $1 being the line number for the next line, and $3 being the
       optional filename (specified with or without quotes).  Note that no
       whitespace may precede the "#", unlike modern C preprocessors.

       There is a fairly obvious gotcha included with the line directive:
       Debuggers and profilers will only show the last source line to appear
       at a particular line number in a given file.  Care should be taken not
       to cause line number collisions in code you'd like to debug later.

       Here are some examples that you should be able to type into your
       command shell:

           % perl
           # line 200 "bzzzt"
           # the '#' on the previous line must be the first char on line
           die 'foo';
           __END__
           foo at bzzzt line 201.

           % perl
           # line 200 "bzzzt"
           eval qq[\n#line 2001 ""\ndie 'foo']; print $@;
           __END__
           foo at - line 2001.

           % perl
           eval qq[\n#line 200 "foo bar"\ndie 'foo']; print $@;
           __END__
           foo at foo bar line 200.

           % perl
           # line 345 "goop"
           eval "\n#line " . __LINE__ . ' "' . __FILE__ ."\"\ndie 'foo'";
           print $@;
           __END__
           foo at goop line 345.

   Experimental Details on given and when
       As previously mentioned, the "switch" feature is considered highly
       experimental; it is subject to change with little notice.  In
       particular, "when" has tricky behaviours that are expected to change to
       become less tricky in the future.  Do not rely upon its current
       (mis)implementation.  Before Perl 5.18, "given" also had tricky
       behaviours that you should still beware of if your code must run on
       older versions of Perl.

       Here is a longer example of "given":

           use feature ":5.10";
           given ($foo) {
               when (undef) {
                   say '$foo is undefined';
               }
               when ("foo") {
                   say '$foo is the string "foo"';
               }
               when ([1,3,5,7,9]) {
                   say '$foo is an odd digit';
                   continue; # Fall through
               }
               when ($_ < 100) {
                   say '$foo is numerically less than 100';
               }
               when (\&complicated_check) {
                   say 'a complicated check for $foo is true';
               }
               default {
                   die q(I don't know what to do with $foo);
               }
           }

       Before Perl 5.18, "given(EXPR)" assigned the value of EXPR to merely a
       lexically scoped copy (!) of $_, not a dynamically scoped alias the way
       "foreach" does.  That made it similar to

               do { my $_ = EXPR; ... }

       except that the block was automatically broken out of by a successful
       "when" or an explicit "break".  Because it was only a copy, and because
       it was only lexically scoped, not dynamically scoped, you could not do
       the things with it that you are used to in a "foreach" loop.  In
       particular, it did not work for arbitrary function calls if those
       functions might try to access $_.  Best stick to "foreach" for that.

       Most of the power comes from the implicit smartmatching that can
       sometimes apply.  Most of the time, "when(EXPR)" is treated as an
       implicit smartmatch of $_, that is, "$_ ~~ EXPR".  (See "Smartmatch
       Operator" in perlop for more information on smartmatching.)  But when
       EXPR is one of the 10 exceptional cases (or things like them) listed
       below, it is used directly as a boolean.

       1.  A user-defined subroutine call or a method invocation.

       2.  A regular expression match in the form of "/REGEX/", "$foo =~
           /REGEX/", or "$foo =~ EXPR".  Also, a negated regular expression
           match in the form "!/REGEX/", "$foo !~ /REGEX/", or "$foo !~ EXPR".

       3.  A smart match that uses an explicit "~~" operator, such as "EXPR ~~
           EXPR".

           NOTE: You will often have to use "$c ~~ $_" because the default
           case uses "$_ ~~ $c" , which is frequentlythe opposite of what you
           want.

       4.  A boolean comparison operator such as "$_ < 10" or "$x eq "abc"".
           The relational operators that this applies to are the six numeric
           comparisons ("<", ">", "<=", ">=", "==", and "!="), and the six
           string comparisons ("lt", "gt", "le", "ge", "eq", and "ne").

       5.  At least the three builtin functions "defined(...)", "exists(...)",
           and "eof(...)".  We might someday add more of these later if we
           think of them.

       6.  A negated expression, whether "!(EXPR)" or "not(EXPR)", or a
           logical exclusive-or, "(EXPR1) xor (EXPR2)".  The bitwise versions
           ("~" and "^") are not included.

       7.  A filetest operator, with exactly 4 exceptions: "-s", "-M", "-A",
           and "-C", as these return numerical values, not boolean ones.  The
           "-z" filetest operator is not included in the exception list.

       8.  The ".." and "..." flip-flop operators.  Note that the "..." flip-
           flop operator is completely different from the "..." elliptical
           statement just described.

       In those 8 cases above, the value of EXPR is used directly as a
       boolean, so no smartmatching is done.  You may think of "when" as a
       smartsmartmatch.

       Furthermore, Perl inspects the operands of logical operators to decide
       whether to use smartmatching for each one by applying the above test to
       the operands:

       9.  If EXPR is "EXPR1 && EXPR2" or "EXPR1 and EXPR2", the test is
           applied recursively to both EXPR1 and EXPR2.  Only if both operands
           also pass the test, recursively, will the expression be treated as
           boolean.  Otherwise, smartmatching is used.

       10. If EXPR is "EXPR1 || EXPR2", "EXPR1 // EXPR2", or "EXPR1 or EXPR2",
           the test is applied recursively to EXPR1 only (which might itself
           be a higher-precedence AND operator, for example, and thus subject
           to the previous rule), not to EXPR2.  If EXPR1 is to use
           smartmatching, then EXPR2 also does so, no matter what EXPR2
           contains.  But if EXPR2 does not get to use smartmatching, then the
           second argument will not be either.  This is quite different from
           the "&&" case just described, so be careful.

       These rules are complicated, but the goal is for them to do what you
       want (even if you don't quite understand why they are doing it).  For
       example:

           when (/^\d+$/ && $_ < 75) { ... }

       will be treated as a boolean match because the rules say both a regex
       match and an explicit test on $_ will be treated as boolean.

       Also:

           when ([qw(foo bar)] && /baz/) { ... }

       will use smartmatching because only one of the operands is a boolean:
       the other uses smartmatching, and that wins.

       Further:

           when ([qw(foo bar)] || /^baz/) { ... }

       will use smart matching (only the first operand is considered), whereas

           when (/^baz/ || [qw(foo bar)]) { ... }

       will test only the regex, which causes both operands to be treated as
       boolean.  Watch out for this one, then, because an arrayref is always a
       true value, which makes it effectively redundant.  Not a good idea.

       Tautologous boolean operators are still going to be optimized away.
       Don't be tempted to write

           when ("foo" or "bar") { ... }

       This will optimize down to "foo", so "bar" will never be considered
       (even though the rules say to use a smartmatch on "foo").  For an
       alternation like this, an array ref will work, because this will
       instigate smartmatching:

           when ([qw(foo bar)] { ... }

       This is somewhat equivalent to the C-style switch statement's
       fallthrough functionality (not to be confused with Perl's fallthrough
       functionality--see below), wherein the same block is used for several
       "case" statements.

       Another useful shortcut is that, if you use a literal array or hash as
       the argument to "given", it is turned into a reference.  So
       "given(@foo)" is the same as "given(\@foo)", for example.

       "default" behaves exactly like "when(1 == 1)", which is to say that it
       always matches.

       Breaking out

       You can use the "break" keyword to break out of the enclosing "given"
       block.  Every "when" block is implicitly ended with a "break".

       Fall-through

       You can use the "continue" keyword to fall through from one case to the
       next immediate "when" or "default":

           given($foo) {
               when (/x/) { say '$foo contains an x'; continue }
               when (/y/) { say '$foo contains a y'            }
               default    { say '$foo does not contain a y'    }
           }

       Return value

       When a "given" statement is also a valid expression (for example, when
       it's the last statement of a block), it evaluates to:

       o   An empty list as soon as an explicit "break" is encountered.

       o   The value of the last evaluated expression of the successful
           "when"/"default" clause, if there happens to be one.

       o   The value of the last evaluated expression of the "given" block if
           no condition is true.

       In both last cases, the last expression is evaluated in the context
       that was applied to the "given" block.

       Note that, unlike "if" and "unless", failed "when" statements always
       evaluate to an empty list.

           my $price = do {
               given ($item) {
                   when (["pear", "apple"]) { 1 }
                   break when "vote";      # My vote cannot be bought
                   1e10  when /Mona Lisa/;
                   "unknown";
               }
           };

       Currently, "given" blocks can't always be used as proper expressions.
       This may be addressed in a future version of Perl.

       Switching in a loop

       Instead of using "given()", you can use a "foreach()" loop.  For
       example, here's one way to count how many times a particular string
       occurs in an array:

           use v5.10.1;
           my $count = 0;
           for (@array) {
               when ("foo") { ++$count }
           }
           print "\@array contains $count copies of 'foo'\n";

       Or in a more recent version:

           use v5.14;
           my $count = 0;
           for (@array) {
               ++$count when "foo";
           }
           print "\@array contains $count copies of 'foo'\n";

       At the end of all "when" blocks, there is an implicit "next".  You can
       override that with an explicit "last" if you're interested in only the
       first match alone.

       This doesn't work if you explicitly specify a loop variable, as in "for
       $item (@array)".  You have to use the default variable $_.

       Differences from Perl 6

       The Perl 5 smartmatch and "given"/"when" constructs are not compatible
       with their Perl 6 analogues.  The most visible difference and least
       important difference is that, in Perl 5, parentheses are required
       around the argument to "given()" and "when()" (except when this last
       one is used as a statement modifier).  Parentheses in Perl 6 are always
       optional in a control construct such as "if()", "while()", or "when()";
       they can't be made optional in Perl 5 without a great deal of potential
       confusion, because Perl 5 would parse the expression

           given $foo {
               ...
           }

       as though the argument to "given" were an element of the hash %foo,
       interpreting the braces as hash-element syntax.

       However, their are many, many other differences.  For example, this
       works in Perl 5:

           use v5.12;
           my @primary = ("red", "blue", "green");

           if (@primary ~~ "red") {
               say "primary smartmatches red";
           }

           if ("red" ~~ @primary) {
               say "red smartmatches primary";
           }

           say "that's all, folks!";

       But it doesn't work at all in Perl 6.  Instead, you should use the
       (parallelizable) "any" operator:

          if any(@primary) eq "red" {
              say "primary smartmatches red";
          }

          if "red" eq any(@primary) {
              say "red smartmatches primary";
          }

       The table of smartmatches in "Smartmatch Operator" in perlop is not
       identical to that proposed by the Perl 6 specification, mainly due to
       differences between Perl 6's and Perl 5's data models, but also because
       the Perl 6 spec has changed since Perl 5 rushed into early adoption.

       In Perl 6, "when()" will always do an implicit smartmatch with its
       argument, while in Perl 5 it is convenient (albeit potentially
       confusing) to suppress this implicit smartmatch in various rather
       loosely-defined situations, as roughly outlined above.  (The difference
       is largely because Perl 5 does not have, even internally, a boolean
       type.)



perl v5.26.1                      2017-07-18                      PERLSYN(1pm)

perl 5.26.1 - Generated Fri Nov 10 05:54:42 CST 2017
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