I can see the interpolation problem, but whats this about not using a constant as a hash key?

#!/usr/bin/perl -w
use strict;

use constant LOG_DIRECTORY => '/foo/bar/baz';

my %hash;
$hash{LOG_DIRECTORY} = 'happy';

print "log directory ${\LOG_DIRECTORY} is $hash{LOG_DIRECTORY}\n";

__END__
produces:

log directory /foo/bar/baz is happy
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-Blake