This is why 'use constant' is bad.

constant.pl

#!/usr/bin/env perl
#
use warnings;
use strict;
use Data::Dumper;

use constant MY_CONSTANT => 13; 

my %foo = (MY_CONSTANT => 'dingle');

print Dumper \%foo;

#yields:
$VAR1 = {
          'MY_CONSTANT' => 'dingle'
        };

Use +MY_CONSTANT or &MY_CONSTANT in the hash key portion. http://perldoc.perl.org/constant.html#CAVEATS