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feature(3perl) Perl Programmers Reference Guide feature(3perl)
NAME
feature - Perl pragma to enable new features
SYNOPSIS
use feature qw(fc say);
# Without the "use feature" above, this code would not be able to find
# the built-ins "say" or "fc":
say "The case-folded version of $x is: " . fc $x;
# set features to match the :5.10 bundle, which may turn off or on
# multiple features (see below)
use feature ':5.10';
# implicitly loads :5.10 feature bundle
use v5.10;
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 Raku-inspired "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: This feature is still experimental and the implementation may change or be
removed 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::smartmatch";
"use feature 'switch'" tells the compiler to enable the Raku 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 "use5.16" 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 "usefeature'evalbytes'"
nor a "usev5.16" (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 supported the legacy $[ variable. See "$[" in perlvar. It was on by default
but disabled under "use v5.16" (see "IMPLICIT LOADING", below) and unavailable since perl
5.30.
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 or be
removed 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 or be
removed 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
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. Starting in Perl 5.28, "use v5.28" will
enable the feature. Before 5.28, it was still experimental and would emit a warning in
the "experimental::bitwise" category.
The 'declared_refs' feature
WARNING: This feature is still experimental and the implementation may change or be
removed 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.
The 'isa' feature
WARNING: This feature is still experimental and the implementation may change or be
removed 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::isa";
This allows the use of the "isa" infix operator, which tests whether the scalar given by
the left operand is an object of the class given by the right operand. See "Class Instance
Operator" in perlop for more details.
This feature is available from Perl 5.32 onwards.
The 'indirect' feature
This feature allows the use of indirect object syntax for method calls, e.g. "new Foo 1,
2;". It is enabled by default, but can be turned off to disallow indirect object syntax.
This feature is available under this name from Perl 5.32 onwards. In previous versions, it
was simply on all the time. To disallow (or warn on) indirect object syntax on older
Perls, see the indirect CPAN module.
The 'multidimensional' feature
This feature enables multidimensional array emulation, a perl 4 (or earlier) feature that
was used to emulate multidimensional arrays with hashes. This works by converting code
like $foo{$x, $y} into $foo{join($;, $x, $y)}. It is enabled by default, but can be
turned off to disable multidimensional array emulation.
When this feature is disabled the syntax that is normally replaced will report a
compilation error.
This feature is available under this name from Perl 5.34 onwards. In previous versions, it
was simply on all the time.
You can use the multidimensional module on CPAN to disable multidimensional array
emulation for older versions of Perl.
The 'bareword_filehandles' feature.
This feature enables bareword filehandles for builtin functions operations, a generally
discouraged practice. It is enabled by default, but can be turned off to disable bareword
filehandles, except for the exceptions listed below.
The perl built-in filehandles "STDIN", "STDOUT", "STDERR", "DATA", "ARGV", "ARGVOUT" and
the special "_" are always enabled.
This feature is enabled under this name from Perl 5.34 onwards. In previous versions it
was simply on all the time.
You can use the bareword::filehandles module on CPAN to disable bareword filehandles for
older versions of perl.
The 'try' feature.
WARNING: This feature is still experimental and the implementation may change or be
removed 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::try";
This feature enables the "try" and "catch" syntax, which allows exception handling, where
exceptions thrown from the body of the block introduced with "try" are caught by executing
the body of the "catch" block.
For more information, see "Try Catch Exception Handling" in perlsyn.
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 indirect multidimensional
bareword_filehandles
:5.10 bareword_filehandles indirect
multidimensional say state switch
:5.12 bareword_filehandles indirect
multidimensional say state switch
unicode_strings
:5.14 bareword_filehandles indirect
multidimensional say state switch
unicode_strings
:5.16 bareword_filehandles current_sub evalbytes
fc indirect multidimensional say state
switch unicode_eval unicode_strings
:5.18 bareword_filehandles current_sub evalbytes
fc indirect multidimensional say state
switch unicode_eval unicode_strings
:5.20 bareword_filehandles current_sub evalbytes
fc indirect multidimensional say state
switch unicode_eval unicode_strings
:5.22 bareword_filehandles current_sub evalbytes
fc indirect multidimensional say state
switch unicode_eval unicode_strings
:5.24 bareword_filehandles current_sub evalbytes
fc indirect multidimensional postderef_qq
say state switch unicode_eval
unicode_strings
:5.26 bareword_filehandles current_sub evalbytes
fc indirect multidimensional postderef_qq
say state switch unicode_eval
unicode_strings
:5.28 bareword_filehandles bitwise current_sub
evalbytes fc indirect multidimensional
postderef_qq say state switch unicode_eval
unicode_strings
:5.30 bareword_filehandles bitwise current_sub
evalbytes fc indirect multidimensional
postderef_qq say state switch unicode_eval
unicode_strings
:5.32 bareword_filehandles bitwise current_sub
evalbytes fc indirect multidimensional
postderef_qq say state switch unicode_eval
unicode_strings
:5.34 bareword_filehandles bitwise current_sub
evalbytes fc indirect multidimensional
postderef_qq say state switch unicode_eval
unicode_strings
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.
Unlike "use feature ":5.12"", saying "use v5.12" (or any higher version) also does the
equivalent of "use strict"; see "use" in perlfunc for details.
perl v5.34.0 2023-11-23 feature(3perl)
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