Yes, perl threads are horribly unstable, and should be avoided for anything real. My guess as to what's causing your problem is that MIME::Parser isn't threadsafe. You may not use any global variables but MIME::Parser could be doing something naughty under the hood. Check the source! However, chances are you won't be able to get threads to do anything worthwhile. Fork is an option, and so is POE(perl object environment). Happy segfaulting!