As has already been hinted at, it isn't necessary to spawn and reap threads during operation, only at startup and shutdown of the service as a whole.

IPC::Semaphore and IPC::SysV (IPC = interprocess communication) contain all the technology you'll need for a master process to communicate with its living children (semaphores, which are part of the operating system, are the most efficient way to manage locking and triggering with shared memory).

See also the System V IPC Chapter of Programming Perl by Larry Wall, Tom Christiansen, and Jon Orwant ISBN 0-596-00027-8 for a nice code example of how to use fork with shared memory and semaphores so that the processes can stay alive and remain in contact for control purposes.