use strict;
use warnings;
use Data::Dumper;
my %foo;
$foo{+length} = 42;
$foo{length} = 'interminable';
print Dumper \%foo;

##</code><code>##

Use of uninitialized value $_ in hash element at test.pl line 5.
$VAR1 = {
          '' => 42,
          'length' => 'interminable'
        };