(Apparently, my original reply to this got lost in today's site crash...)

I don't think you need to use anything special like a tie or otherwise for this. The hash key can be an array (not an arrayref), which you can use to emulate multi-key'd hashes without the need for any special coding. See my node An interesting oddity with hashes for more details.

my %hash;
$hash{ 3,4 } = "array";
$hash{ 3 } = "three";
$hash{ 4 } = "four";    # no collisions!
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Dr. Michael K. Neylon - mneylon-pm@masemware.com || "You've left the lens cap of your mind on again, Pinky" - The Brain