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Andy Lester > Perl-Critic > Perl::Critic::Policy::Subroutines::RequireArgUnpacking



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Perl::Critic::Policy::Subroutines::RequireArgUnpacking - Always unpack @_ first.


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


Subroutines that use @_ directly instead of unpacking the arguments to local variables first have two major problems. First, they are very hard to read. If you're going to refer to your variables by number instead of by name, you may as well be writing assembler code! Second, @_ contains aliases to the original variables! If you modify the contents of a @_ entry, then you are modifying the variable outside of your subroutine. For example:

   sub print_local_var_plus_one {
       my ($var) = @_;
       print ++$var;
   sub print_var_plus_one {
       print ++$_[0];

   my $x = 2;
   print_local_var_plus_one($x); # prints "3", $x is still 2
   print_var_plus_one($x);       # prints "3", $x is now 3 !
   print $x;                     # prints "3"

This is spooky action-at-a-distance and is very hard to debug if it's not intentional and well-documented (like chop or chomp).

An exception is made for the usual delegation idiom $object->SUPER::something( @_ ). Only SUPER:: and NEXT:: are recognized (though this is configurable) and the argument list for the delegate must consist only of ( @_ ).


This policy is lenient for subroutines which have N or fewer top-level statements, where N defaults to ZERO. You can override this to set it to a higher number with the short_subroutine_statements setting. This is very much not recommended but perhaps you REALLY need high performance. To do this, put entries in a .perlcriticrc file like this:

  short_subroutine_statements = 2

By default this policy does not allow you to specify array subscripts when you unpack arguments (i.e. by an array slice or by referencing individual elements). Should you wish to permit this, you can do so using the allow_subscripts setting. This defaults to false. You can set it true like this:

  allow_subscripts = 1

The delegation logic can be configured to allow delegation other than to SUPER:: or NEXT::. The configuration item is allow_delegation_to, and it takes a space-delimited list of allowed delegates. If a given delegate ends in a double colon, anything in the given namespace is allowed. If it does not, only that subroutine is allowed. For example, to allow next::method from Class::C3 and _delegate from the current namespace in addition to SUPER and NEXT, the following configuration could be used:

  allow_delegation_to = next::method _delegate


PPI doesn't currently detect anonymous subroutines, so we don't check those. This should just work when PPI gains that feature.

We don't check for @ARG, the alias for @_ from That's deprecated anyway.


Initial development of this policy was supported by a grant from the Perl Foundation.


Chris Dolan <>


Copyright (c) 2007-2011 Chris Dolan. Many rights reserved.

This program is free software; you can redistribute it and/or modify it under the same terms as Perl itself. The full text of this license can be found in the LICENSE file included with this module

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