> Somewhere deep within the thread

If you reproduce it, post the code here. I am guessing you're making some wrong assumption about threads and how they work in perl; without knowing what you're doing, I can blindly suggest: 1) get it working serially, and 2) if you need to workshare, use fork or better, Parallel::ForkManager. Also, if you're reading in a entire 1 TB file or grabbing a billion rows from mysqld, then there are better ways to do it.