This is on AIX. I do call wait_all_children() at the end but due to the way I'm performing work and receiving new work I'm neither waiting on $pm->start nor wait_all_children() so waitpid() in P:FM isn't called. I don't have new work for my classic for() loop until children finish and run_on_finish() is called. Basically run_on_finish() is my only way to get new work.

My code does not call wait() or waitpid(). I don't think I can post my code but let me see if I can get a reproducible small test case.