I apologise

The value passed to $fork_mgr->start() can be used to uniquely identify the thread, and it comes back as $child_id in the subroutine defined in $fork_mgr->run_on_finish()

Given that ID, you can reconcile the work done by the worker threads. In the past I have used that ID for an array index into the array of work to be done, A string that uniquely identifies the work, or the primary key of a database row.