SYNOPSIS

use 5.010;
use lexical::underscore;
use Test::More;

sub is_uppercase {
my $var = @_ ? shift : ${lexical::underscore()};
return $var eq uc($var);
}

my $thing = 'FOO';
my $works = 0;

given ( $thing ) {
when ( is_uppercase ) { $works++ }
}

ok($works);
done_testing();

DESCRIPTION

Starting with Perl 5.10, it is possible to create a lexical version of the Perl default variable $_. Certain Perl constructs like the "given" keyword automatically use a lexical $_ rather than the global $_.

It is occasionallly useful for a sub to be able to access its caller's $_ variable regardless of whether it was lexical or not. The "(_)" sub prototype is the official way to do so, however there are sometimes disadvantages to this; in particular it can only appear as the final required argument in a prototype, and there is no way of the sub differentiating between an explicitly passed argument and $_.

This caused me problems with Scalar::Does, because I wanted to enable the "does" function to be called as either:

   does($thing, $role);
   does($role);  # assumes $thing = $_

With "_" in the prototype, $_ was passed to the function at the end of its argument list; effectively "does($role, $thing)", making it impossible to tell which argument was the role.

Enter "lexical::underscore" which allows you to access your caller's lexical $_ variable as easily as:

   ${lexical::underscore()}

You can access lexical $_ further up the call stack using:

   ${lexical::underscore($level)}

If you happen to ask for $_ at a level where no lexical $_ is available, you get the global $_ instead.

This module does work on Perl 5.8 but as there is no lexical $_, always returns the global $_.

Technical Details

The "lexical::underscore" function returns a scalar reference to either a lexical $_ variable somewhere up the call stack (using PadWalker magic), or to the global $_ if there was no lexical version.

Wrapping "lexical::underscore" in "${ ... }" dereferences the scalar reference, allowing you to access (and even assign to) it.

AUTHOR

Toby Inkster <[email protected]>.

COPYRIGHT AND LICENCE

This software is copyright (c) 2012, 2014 by Toby Inkster.

This is free software; you can redistribute it and/or modify it under the same terms as the Perl 5 programming language system itself.

DISCLAIMER OF WARRANTIES

THIS PACKAGE IS PROVIDED ``AS IS'' AND WITHOUT ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, WITHOUT LIMITATION, THE IMPLIED WARRANTIES OF MERCHANTIBILITY AND FITNESS FOR A PARTICULAR PURPOSE.