in reply to finding longest common substring
It seems I was a bit hasty in posting this question, and probably should have done a little more research first. gmax has pointed out Longest Common Substring, and etcshadow has pointed out Algorithm::Diff (which I have seen before, and is undoubtedly the reason the phrase "longest common substring" was floating around in my brain to begin with). Also, I updated my node mentioning japhy's Longest common substring, but apparently the update got eaten. (Is it just my imagination, or didn't we recently gain the ability to update root nodes?)
While Algorithm::Diff doesn't do exactly the same thing I am doing, it would have probably been useful at least. Sorry for wasting any time, and many thanks for the replies that did come! :-)