So my "sane" options are singular: to extend a given stable wheel (Net::Server?) via some sort of data/state sharing mechanism so that when any given listener is presented with a task ID, that it is able to retrieve the output of that task beginning from the last time it was polled.

Problem is, you've bought into the fallacy that the behemoth that is Net::Server is going to solve some high proportion of your problem. It won't.

The very essence of your project is the problem of routing multiple discrete client connects back to the sources of their data. Handling the connects (the only bit that Net::Server will do for you) is the easy part. Plumbing the appropriate clients to their appropriate data sources is a plumbing problem that Net::Server has no solution to. And using a forking solution for that means you are going to be forced into a multiplexing nightmare.

Which is just silly when -- by your own statement -- you only need ~20 clients, which takes just 36MB for 21 threads:

perl -Mthreads=stack_size,4096 -MIO::Socket -E"async{ IO::Socket::INET
+->new( 'localhost:' . 12300 + threads->tid ); sleep }->detach for 1 .
+. 20; sleep"
[download]

A bit more for some shared memory and all your routing problems become trivial.

C'est la vie :)