Almost:

You are free to use a new copy of Parallel::ForkManager in the child processes, although I don't think it makes sense.

What you saw was:

If you want to use more than one copies of the Parallel::ForkManager, then you have to make sure that all children processes are terminated, before you use the second object in the main program.

However, I would test it to double check. Using the run_on_finish callback would work. Whether it would be more clear or easier to read is up to the user.