I can't imagine many cases at all where you might want to use length $_ as an index or key, but almost every case where you would want to be able to type a key without having quotes all over the place.

The simplest way to do what you want is with a leading unary + to make it not be a bareword.

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

Gives:

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