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feature(3pm)           Perl Programmers Reference Guide           feature(3pm)




NAME

       feature - Perl pragma to enable new features


SYNOPSIS

           use feature qw(say switch);
           given ($foo) {
               when (1)          { say "\$foo == 1" }
               when ([2,3])      { say "\$foo == 2 || \$foo == 3" }
               when (/^a[bc]d$/) { say "\$foo eq 'abd' || \$foo eq 'acd'" }
               when ($_ > 100)   { say "\$foo > 100" }
               default           { say "None of the above" }
           }

           use feature ':5.10'; # loads all features available in perl 5.10

           use v5.10;           # implicitly loads :5.10 feature bundle


DESCRIPTION

       It is usually impossible to add new syntax to Perl without breaking
       some existing programs.  This pragma provides a way to minimize that
       risk. New syntactic constructs, or new semantic meanings to older
       constructs, can be enabled by "use feature 'foo'", and will be parsed
       only when the appropriate feature pragma is in scope.  (Nevertheless,
       the "CORE::" prefix provides access to all Perl keywords, regardless of
       this pragma.)

   Lexical effect
       Like other pragmas ("use strict", for example), features have a lexical
       effect.  "use feature qw(foo)" will only make the feature "foo"
       available from that point to the end of the enclosing block.

           {
               use feature 'say';
               say "say is available here";
           }
           print "But not here.\n";

   "no feature"
       Features can also be turned off by using "no feature "foo"".  This too
       has lexical effect.

           use feature 'say';
           say "say is available here";
           {
               no feature 'say';
               print "But not here.\n";
           }
           say "Yet it is here.";

       "no feature" with no features specified will reset to the default
       group.  To disable all features (an unusual request!) use "no feature
       ':all'".


AVAILABLE FEATURES

   The 'say' feature
       "use feature 'say'" tells the compiler to enable the Perl 6 style "say"
       function.

       See "say" in perlfunc for details.

       This feature is available starting with Perl 5.10.

   The 'state' feature
       "use feature 'state'" tells the compiler to enable "state" variables.

       See "Persistent Private Variables" in perlsub for details.

       This feature is available starting with Perl 5.10.

   The 'switch' feature
       WARNING: Because the smartmatch operator is experimental, Perl will
       warn when you use this feature, unless you have explicitly disabled the
       warning:

           no warnings "experimental::smartmatch";

       "use feature 'switch'" tells the compiler to enable the Perl 6
       given/when construct.

       See "Switch Statements" in perlsyn for details.

       This feature is available starting with Perl 5.10.

   The 'unicode_strings' feature
       "use feature 'unicode_strings'" tells the compiler to use Unicode rules
       in all string operations executed within its scope (unless they are
       also within the scope of either "use locale" or "use bytes").  The same
       applies to all regular expressions compiled within the scope, even if
       executed outside it.  It does not change the internal representation of
       strings, but only how they are interpreted.

       "no feature 'unicode_strings'" tells the compiler to use the
       traditional Perl rules wherein the native character set rules is used
       unless it is clear to Perl that Unicode is desired.  This can lead to
       some surprises when the behavior suddenly changes.  (See "The "Unicode
       Bug"" in perlunicode for details.)  For this reason, if you are
       potentially using Unicode in your program, the "use feature
       'unicode_strings'" subpragma is strongly recommended.

       This feature is available starting with Perl 5.12; was almost fully
       implemented in Perl 5.14; and extended in Perl 5.16 to cover
       "quotemeta"; and extended further in Perl 5.26 to cover the range
       operator.

   The 'unicode_eval' and 'evalbytes' features
       Together, these two features are intended to replace the legacy string
       "eval" function, which behaves problematically in some instances.  They
       are available starting with Perl 5.16, and are enabled by default by a
       "use 5.16" or higher declaration.

       "unicode_eval" changes the behavior of plain string "eval" to work more
       consistently, especially in the Unicode world.  Certain (mis)behaviors
       couldn't be changed without breaking some things that had come to rely
       on them, so the feature can be enabled and disabled.  Details are at
       "Under the "unicode_eval" feature" in perlfunc.

       "evalbytes" is like string "eval", but operating on a byte stream that
       is not UTF-8 encoded.  Details are at "evalbytes EXPR" in perlfunc.
       Without a "use feature 'evalbytes'" nor a "use v5.16" (or higher)
       declaration in the current scope, you can still access it by instead
       writing "CORE::evalbytes".

   The 'current_sub' feature
       This provides the "__SUB__" token that returns a reference to the
       current subroutine or "undef" outside of a subroutine.

       This feature is available starting with Perl 5.16.

   The 'array_base' feature
       This feature supports the legacy $[ variable.  See "$[" in perlvar and
       arybase.  It is on by default but disabled under "use v5.16" (see
       "IMPLICIT LOADING", below).

       This feature is available under this name starting with Perl 5.16.  In
       previous versions, it was simply on all the time, and this pragma knew
       nothing about it.

   The 'fc' feature
       "use feature 'fc'" tells the compiler to enable the "fc" function,
       which implements Unicode casefolding.

       See "fc" in perlfunc for details.

       This feature is available from Perl 5.16 onwards.

   The 'lexical_subs' feature
       In Perl versions prior to 5.26, this feature enabled declaration of
       subroutines via "my sub foo", "state sub foo" and "our sub foo" syntax.
       See "Lexical Subroutines" in perlsub for details.

       This feature is available from Perl 5.18 onwards.  From Perl 5.18 to
       5.24, it was classed as experimental, and Perl emitted a warning for
       its usage, except when explicitly disabled:

         no warnings "experimental::lexical_subs";

       As of Perl 5.26, use of this feature no longer triggers a warning,
       though the "experimental::lexical_subs" warning category still exists
       (for compatibility with code that disables it).  In addition, this
       syntax is not only no longer experimental, but it is enabled for all
       Perl code, regardless of what feature declarations are in scope.

   The 'postderef' and 'postderef_qq' features
       The 'postderef_qq' feature extends the applicability of postfix
       dereference syntax so that postfix array and scalar dereference are
       available in double-quotish interpolations. For example, it makes the
       following two statements equivalent:

         my $s = "[@{ $h->{a} }]";
         my $s = "[$h->{a}->@*]";

       This feature is available from Perl 5.20 onwards. In Perl 5.20 and
       5.22, it was classed as experimental, and Perl emitted a warning for
       its usage, except when explicitly disabled:

         no warnings "experimental::postderef";

       As of Perl 5.24, use of this feature no longer triggers a warning,
       though the "experimental::postderef" warning category still exists (for
       compatibility with code that disables it).

       The 'postderef' feature was used in Perl 5.20 and Perl 5.22 to enable
       postfix dereference syntax outside double-quotish interpolations. In
       those versions, using it triggered the "experimental::postderef"
       warning in the same way as the 'postderef_qq' feature did. As of Perl
       5.24, this syntax is not only no longer experimental, but it is enabled
       for all Perl code, regardless of what feature declarations are in
       scope.

   The 'signatures' feature
       WARNING: This feature is still experimental and the implementation may
       change in future versions of Perl.  For this reason, Perl will warn
       when you use the feature, unless you have explicitly disabled the
       warning:

           no warnings "experimental::signatures";

       This enables unpacking of subroutine arguments into lexical variables
       by syntax such as

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

       See "Signatures" in perlsub for details.

       This feature is available from Perl 5.20 onwards.

   The 'refaliasing' feature
       WARNING: This feature is still experimental and the implementation may
       change in future versions of Perl.  For this reason, Perl will warn
       when you use the feature, unless you have explicitly disabled the
       warning:

           no warnings "experimental::refaliasing";

       This enables aliasing via assignment to references:

           \$a = \$b; # $a and $b now point to the same scalar
           \@a = \@b; #                     to the same array
           \%a = \%b;
           \&a = \&b;
           foreach \%hash (@array_of_hash_refs) {
               ...
           }

       See "Assigning to References" in perlref for details.

       This feature is available from Perl 5.22 onwards.

   The 'bitwise' feature
       WARNING: This feature is still experimental and the implementation may
       change in future versions of Perl.  For this reason, Perl will warn
       when you use the feature, unless you have explicitly disabled the
       warning:

           no warnings "experimental::bitwise";

       This makes the four standard bitwise operators ("& | ^ ~") treat their
       operands consistently as numbers, and introduces four new dotted
       operators ("&. |. ^. ~.") that treat their operands consistently as
       strings.  The same applies to the assignment variants ("&= |= ^= &.=
       |.= ^.=").

       See "Bitwise String Operators" in perlop for details.

       This feature is available from Perl 5.22 onwards.

   The 'declared_refs' feature
       WARNING: This feature is still experimental and the implementation may
       change in future versions of Perl.  For this reason, Perl will warn
       when you use the feature, unless you have explicitly disabled the
       warning:

           no warnings "experimental::declared_refs";

       This allows a reference to a variable to be declared with "my",
       "state", our "our", or localized with "local".  It is intended mainly
       for use in conjunction with the "refaliasing" feature.  See "Declaring
       a Reference to a Variable" in perlref for examples.

       This feature is available from Perl 5.26 onwards.


FEATURE BUNDLES

       It's possible to load multiple features together, using a feature
       bundle.  The name of a feature bundle is prefixed with a colon, to
       distinguish it from an actual feature.

         use feature ":5.10";

       The following feature bundles are available:

         bundle    features included
         --------- -----------------
         :default  array_base

         :5.10     say state switch array_base

         :5.12     say state switch unicode_strings array_base

         :5.14     say state switch unicode_strings array_base

         :5.16     say state switch unicode_strings
                   unicode_eval evalbytes current_sub fc

         :5.18     say state switch unicode_strings
                   unicode_eval evalbytes current_sub fc

         :5.20     say state switch unicode_strings
                   unicode_eval evalbytes current_sub fc

         :5.22     say state switch unicode_strings
                   unicode_eval evalbytes current_sub fc

         :5.24     say state switch unicode_strings
                   unicode_eval evalbytes current_sub fc
                   postderef_qq

         :5.26     say state switch unicode_strings
                   unicode_eval evalbytes current_sub fc
                   postderef_qq

       The ":default" bundle represents the feature set that is enabled before
       any "use feature" or "no feature" declaration.

       Specifying sub-versions such as the 0 in 5.14.0 in feature bundles has
       no effect.  Feature bundles are guaranteed to be the same for all sub-
       versions.

         use feature ":5.14.0";    # same as ":5.14"
         use feature ":5.14.1";    # same as ":5.14"


IMPLICIT LOADING

       Instead of loading feature bundles by name, it is easier to let Perl do
       implicit loading of a feature bundle for you.

       There are two ways to load the "feature" pragma implicitly:

       o   By using the "-E" switch on the Perl command-line instead of "-e".
           That will enable the feature bundle for that version of Perl in the
           main compilation unit (that is, the one-liner that follows "-E").

       o   By explicitly requiring a minimum Perl version number for your
           program, with the "use VERSION" construct.  That is,

               use v5.10.0;

           will do an implicit

               no feature ':all';
               use feature ':5.10';

           and so on.  Note how the trailing sub-version is automatically
           stripped from the version.

           But to avoid portability warnings (see "use" in perlfunc), you may
           prefer:

               use 5.010;

           with the same effect.

           If the required version is older than Perl 5.10, the ":default"
           feature bundle is automatically loaded instead.



perl v5.26.1                      2017-07-18                      feature(3pm)

perl 5.26.1 - Generated Tue Nov 7 07:39:42 CST 2017
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