To be honest, I can't understand what you are doing in your code, with the goto LOOP and the seek. Your first step should be to see how many threads are being created, you may have a thread bomb in your code. How many threads are reportedly created by the printout of $j, and how much memory are you consuming. Try watching the script with top, and see how much memory you use.

You should strip down your code, to the point where you are just creating and joining threads which do nothing. When you get that to work, start adding your math routines, and see where it starts to break down.