liverpole,
I didn't read your entire post, just a couple of lines.

Having said that, though, there is one exception: an empty string will NOT work,...

I am not sure where you got that idea but an empty string works fine:

#!/usr/bin/perl
use strict;
use warnings;
my %hash;
$hash{''} = 'foo';
print $hash{''};
[download]

But I'm still very curious -- how on earth did such a question occur to you?

My guess would be due to a background in another language. Certain C libraries have restriction on hash key length for instance while Java allows you to store objects as hash keys .