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


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


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::MoreUtils::apply().


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

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

    list_funcs = map grep List::Util::first

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

    add_list_funcs = Foo::Bar::listmunge


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

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

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

and not like this:

    use List::MoreUtils 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.


There is discussion of this policy at


Chris Dolan <>

Michael Wolf <>


Copyright (c) 2006-2011 Chris Dolan.

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