To nit, scalar keys %hash is probably much more easier to understand and may be faster as well (as you don't have to redeclare a new anon array.

I've been striving to move away from "$#array" to do similar counts in favor of "scalar @array - 1", as the latter is much easier to read, and while I know that perl does not treat the # in $# as a comment starter, I've seen people get confused by that.

Yes, there are times where 'cute' tricks work better and may help others to improve their programming skills (for example, using map or grep instead of for loops for some tasks), but there are others that border on obfuscation, and should be kept there. I wouldn't call the trick above strictly obfu, but it's not readily understandable and as you say, your super went away head scratching; you gave him a fish, but didn't show him how you got it.

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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
"I can see my house from here!"
It's not what you know, but knowing how to find it if you don't know that's important