The on_finish callback is only called when P::FM reaps a child, and P::FM only reaps a child under three conditions:

• When $pm->start is called and the number of children that have been started but not reaped is equal to the maximum.
• When $pm->reap_finished_children is called.
• When $pm->wait_all_children is called.

There could be an arbitrarily long delay between a child exiting and one the above events. Adding the following to your program should eliminate that delay:

$SIG{CHLD} = sub { $pm->reap_finished_children };
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By the way, if the work performed by your child only takes "a few milliseconds or nanoseconds", you are actually slowing things down by using P::FM. Data passed to `finish` gets serialized and written to disk, then read from the disk and deserialized for the `on_finish` callback!