Perl::Critic::Policy::ControlStructures::ProhibitMutatingListFunctions - Don't modify "$_" in list

       This Policy is part of the core Perl::Critic distribution.

       Chris Dolan <cdolan@cpan.org>

       Michael Wolf <MichaelRWolf@att.net>

       By default, this policy applies to the following list functions:

           map grep
           List::Util qw(first)
           List::MoreUtils qw(any all none notall true false firstidx
                              first_index lastidx last_index insert_after
                              insert_after_string)
           List::SomeUtils qw(any all none notall true false firstidx
                              first_index lastidx last_index insert_after
                              insert_after_string)

       This list can be overridden the .perlcriticrc file like this:

           [ControlStructures::ProhibitMutatingListFunctions]
           list_funcs = map grep List::Util::first

       Or, one can just append to the list like so:

           [ControlStructures::ProhibitMutatingListFunctions]
           add_list_funcs = Foo::Bar::listmunge

       Copyright (c) 2006-2021 Chris Dolan.

       This program is free software; you can redistribute it and/or modify it under the same terms as Perl
       itself.

perl v5.40.0                                       2024-10-28             Perl::Critic::P...ngListFunctions(3pm)

       "map", "grep" and other list operators are intended to transform arrays into other arrays by applying
       code to the array elements one by one.  For speed, the elements are referenced via a $_ alias rather than
       copying them.  As a consequence, if the code block of the "map" or "grep" modify $_ in any way, then it
       is actually modifying the source array.  This IS technically allowed, but those side effects can be quite
       surprising, especially when the array being passed is @_ or perhaps values(%ENV)!  Instead authors should
       restrict in-place array modification to "for(@array) { ... }" constructs instead, or use
       List::SomeUtils:apply() or List::MoreUtils::apply().

       This policy deliberately does not apply to "for (@array) { ... }" or List::MoreUtils::apply()
       List::SomeUtils::apply().

       Currently, the policy only detects explicit external module usage like this:

           my @out = List::MoreUtils::any {s/^foo//} @in;
           my @out = List::SomeUtils::any {s/^foo//} @in;

       and not like this:

           use List::MoreUtils qw(any);
           my @out = any {s/^foo//} @in;

           use List::SomeUtils qw(any);
           my @out = any {s/^foo//} @in;

       This policy looks only for modifications of $_.  Other naughtiness could include modifying $a and $b in
       "sort" and the like.  That's beyond the scope of this policy.

       Perl::Critic::Policy::ControlStructures::ProhibitMutatingListFunctions - Don't modify "$_" in list
       functions.

       There is discussion of this policy at <http://perlmonks.org/index.pl?node_id=743445>.