hmm although I prefer fork to thread so as to surivive later versions of perl, this does give me an idea of how to implement my own threads using fork in a way that overcomes my filehandle problem with the standard drop-in solution:-

Since I know in advance I am going to use the max configured subprocesses given that there are 150000 jobs in the queue being rapidly thrown at my scheduling architecture, I could start by forking precisely that number of subprocesses using open |- and let the children live to the very end, sending the code they have to manage over the pipe.

update: but then whether I do that or use your queued thread approach, I need also to read back from the child in order to perform complicated load-balancing. If the subprocesses are allowed to die per iteration (i.e per job parsed and submitted to a child or thread) I wouldn't have that problem