If I could clean up the files to just have the lines I was interested in comparing, then using DBD::CSV would probably just be a query like "SELECT * FROM FILE1 WHERE IP NOT IN (SELECT IP FROM FILE2)". Or something like that. Now, granted, DBD::CSV does take a bit more to set up. But once you have it set up, there's just so much more you can do with SQL with so little code that it may be worth it.

I was hoping, though, that I could see a way to get DBD::CSV to read from a filehandle rather than a file, so I could use an in-memory file as a demo, but I don't see that. Oh well.