That is something I would like to learn myself. :-) If you have perl and Tk compiled with debugging support, and you havn't stripped them, you can run start perl under gdb, then run the Tk program, and watch for what happens. I'm only rudimentarily familiar with doing it, so you might want to google for "gdb perl Tk" or "debugging perl tk", and see what you can come up with.

Maybe you could run it under ptkdb, and see what happens when you step thru it, but with threads( and Tk not being thread-safe), the error will be "weird" and may not follow a repeatable pattern. It will just happen seemingly at random times.