Already asked and answered on StackOverflow. You just have not accepted the answer. Bad form to cross post and not mention it.

Summary: Parallel::ForkManager reaps its processes when the number of processes has reached the max and you try to start a new one. Since you are in an infinite loop, there is always the 'last' process that remains unreaped until a new one starts (all of them remain unreaped until you start process max+1). If your loop would finish, you can call wait_all_children() and that last process will be reaped when it finishes. That is how P::FM works. If you want different behaviour, use something else.

Also, why is this a problem? You seem to be under the misapprehension that having one zombie process temporarily hanging around is bad. If your process let many such processes accumulate, that might become a problem. But that does not seem to be the case...so, what is the problem?