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"; was extended further in Perl 5.26 to cover the range
operator; and was extended again in Perl 5.28 to cover special-cased
whitespace splitting.
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 v5.26.3 2023-05-18 feature(3pm)
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