Yes, I have heard about issues with multi-threading and Perl 5.8. Whether or not they resemble yours I can only guess.

The following discussions give some perspective on the state of Perl's threading: Why use threads over processes, or why use processes over threads?, Perl 5.8.2 thread is much worse and My experience with Perl threading and the reason I think that I have rushed a little bit. If you Super Search, you can find others.

My suggestion is to avoid multi-threading unless you have a really compelling reason to use it.