Can't you use your lock file to store this information? Of course you will need to keep track of every birth and death. Another possible approach is to drop a file every time you start a process in a predefined directory and count them to know how many processes are running. This kind of counter can use the timestamp of files to know if a process aborted (you need to simulate an heartbeat, updating mtime of your files periodically).
There is also Only let one copy of program run at a time, published today by esh, and Parallel::ForkManager.

Ciao, Valerio