First off, let me say many thanks to everyone who gave me feedback on this problem, with just six replies I've gained not only a solution to my problem, but also jumped a couple of steps forward in my benchmarking, debugging, and just coding in general.

Now the specifics:

1. First off, graff was absolutely correct, the second half of the source/dest regexp should've been: ((?:\d{1,3}\.){3}\d{1,3})
But all things considered, I think I'll be storing smaller chunks of regexps in variables like graff shows, (read as: All along I should've been storing smaller...)

2. It seems the root of all my troubles was the memory intensive file-slurp... the process was originally designed to run on LWP::UserAgent->content returns, but was later modified to work on the local filesystem instead, I never modded the structure to be able to use line-by-line mode. I was uselessly throwing away resources which was the source of all my troubles.