experimental(3perl) Perl Programmers Reference Guide experimental(3perl)
NAME
experimental - Experimental features made easy
VERSION
version 0.032
SYNOPSIS
use experimental 'lexical_subs', 'signatures';
my sub plus_one($value) { $value + 1 }
DESCRIPTION
This pragma provides an easy and convenient way to enable or disable
experimental features.
Every version of perl has some number of features present but considered
"experimental." For much of the life of Perl 5, this was only a
designation found in the documentation. Starting in Perl v5.10.0, and
more aggressively in v5.18.0, experimental features were placed behind
pragmata used to enable the feature and disable associated warnings.
The "experimental" pragma exists to combine the required incantations
into a single interface stable across releases of perl. For every
experimental feature, this should enable the feature and silence
warnings for the enclosing lexical scope:
use experimental 'feature-name';
To disable the feature and, if applicable, re-enable any warnings, use:
no experimental 'feature-name';
The supported features, documented further below, are:
• "args_array_with_signatures" - allow @_ to be used in signatured
subs.
This is supported on perl 5.20.0 and above, but is likely to be
removed in the future.
• "array_base" - allow the use of $[ to change the starting index of
@array.
This was removed in perl 5.30.0.
• "autoderef" - allow push, each, keys, and other built-ins on
references.
This was added in perl 5.14.0 and removed in perl 5.24.0.
• "bitwise" - allow the new stringwise bit operators
This was added in perl 5.22.0.
• "builtin" - allow the use of the functions in the builtin::
namespace
This was added in perl 5.36.0
• "const_attr" - allow the :const attribute on subs
This was added in perl 5.22.0.
• "declared_refs" - enables aliasing via assignment to references
This was added in perl 5.26.0.
• "defer" - enables the use of defer blocks
This was added in perl 5.36.0
• "extra_paired_delimiters" - enables the use of more paired string
delimiters than the traditional four, "< >", "( )", "{ }", and
"[ ]".
This was added in perl 5.36.
• "for_list" - allows iterating over multiple values at a time with
"for"
This was added in perl 5.36.0
• "isa" - allow the use of the "isa" infix operator
This was added in perl 5.32.0.
• "lexical_topic" - allow the use of lexical $_ via "my $_".
This was added in perl 5.10.0 and removed in perl 5.24.0.
• "lexical_subs" - allow the use of lexical subroutines.
This was added in 5.18.0, and became non-experimental (and always
enabled) in 5.26.0.
• "postderef" - allow the use of postfix dereferencing expressions
This was added in perl 5.20.0, and became non-experimental (and
always enabled) in 5.24.0.
• "postderef_qq" - allow the use of postfix dereferencing expressions
inside interpolating strings
This was added in perl 5.20.0, and became non-experimental (and
always enabled) in 5.24.0.
• "re_strict" - enables strict mode in regular expressions
This was added in perl 5.22.0.
• "refaliasing" - allow aliasing via "\$x = \$y"
This was added in perl 5.22.0.
• "regex_sets" - allow extended bracketed character classes in regexps
This was added in perl 5.18.0.
• "signatures" - allow subroutine signatures (for named arguments)
This was added in perl 5.20.0.
• "smartmatch" - allow the use of "~~"
This was added in perl 5.10.0, but it should be noted there are
significant incompatibilities between 5.10.0 and 5.10.1.
The feature is going to be deprecated in perl 5.38.0, and removed in
5.42.0.
• "switch" - allow the use of "~~", given, and when
This was added in perl 5.10.0.
The feature is going to be deprecated in perl 5.38.0, and removed in
5.42.0.
• "try" - allow the use of "try" and "catch"
This was added in perl 5.34.0
• "win32_perlio" - allows the use of the :win32 IO layer.
This was added on perl 5.22.0.
Ordering matters
Using this pragma to 'enable an experimental feature' is another way of
saying that this pragma will disable the warnings which would result
from using that feature. Therefore, the order in which pragmas are
applied is important. In particular, you probably want to enable
experimental features after you enable warnings:
use warnings;
use experimental 'smartmatch';
You also need to take care with modules that enable warnings for you. A
common example being Moose. In this example, warnings for the
'smartmatch' feature are first turned on by the warnings pragma, off by
the experimental pragma and back on again by the Moose module (fix is to
switch the last two lines):
use warnings;
use experimental 'smartmatch';
use Moose;
Disclaimer
Because of the nature of the features it enables, forward compatibility
can not be guaranteed in any way.
SEE ALSO
perlexperiment contains more information about experimental features.
AUTHOR
Leon Timmermans <leont@cpan.org>
COPYRIGHT AND LICENSE
This software is copyright (c) 2013 by Leon Timmermans.
This is free software; you can redistribute it and/or modify it under
the same terms as the Perl 5 programming language system itself.
perl v5.40.1 2026-08-30 experimental(3perl)
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