Just a quick note that globals get destroyed in somewhat random order and that can often result in references turning into undef in the middle of "global destruction". Actually, they are still references (because ref returns a true value), they just have an address of 0 (they return 0 in a numeric context), which Perl reports as "undef" when you actually try to use it.

I wish Perl would at least have an option to, during clean up, decrement the ref count on all globals, destroying things in the normal manner, and after that, would then go sweep up anything that was left over.

That only partially addresses one of your questions, but that was all I have time for at the moment. (:

Update: Hmm, you aren't using any globals so this probably isn't the problem, though. Hmm...

Oh, wait. You have hard-coded references to your lexical %x in your subroutines. This means that %x won't be freed until those subroutines are freed and subroutines don't get freed until "the end". Change your code so that \%x is passed in to TIEHASH (and keep the reference to it commented out in DESTROY) and see how that changes things.