I think the problem is that you're hoping for too much intelligence on the part of =>. When perl (Perl?) sees the bare '#', it assumes it's an end-of-line comment. If, however, you quote the key, it works fine.

#!/usr/bin/perl -w

use strict;

my %hash = ( partname  => "large widget",
              'cata#ofitem' => "4321"
           );

print $hash{ 'cata#ofitem' }, "\n";
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correctly reports "4321" as expected.