In addition to those problems, there's also my very first post here on PM. It was about a problem with tied hashes...to which I never really got a satisfactory response: Whither the truth of a tied hash?

In short, a populated hash will respond truthfully in a scalar context:

if (%h) { ...do something... }
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whereas a populated tied hash will do no such thing. It will always evaluate to false.

So much for drop-in hash sniffers.

Matt