I actually just figured this out about an hour ago.  Add 

waitpid $pid, 0;

to your script when you are done with each individual call 
to IO::Pipe.  That frees up the resource that is hanging 
onto a dead process.open2 can only handle a limited number 
of processes (128 or 64), so you have to unleash the process 
by waiting for it to close itself (waitpid).