In windows it's all a big experiment ... turn wifi off, updates off, email, browsers, a neverending catch-me-if-you-can with the phone-home fairy.

Anyway, I have no idea on the internals of Perl regexes. It looks like they are the only non-deterministic element in your code, with the hash key search. And, in theory at least, they can contribute that variability in time you observed. Perhaps. It's worth checking min, max and stdev runtime of each of your regexes.