There is only one cause for the creation of a zombie process: the parent process hasn't collected a child process exit status via waitpid or set $SIG{CHLD} = 'IGNORE' (this signals your program's lack of interest in its offspring). In other words, if your program doesn't explicitly ignore SIGCHLD, all its child processes will hang around like the undead until waitpid is called for each one. If you look at the perldoc for Proc::Fork, the synopsis shows the basic format for creating and disposing of child processes, and the waitpid call is clearly shown. If your program really doesn't make zombies under 5.6.1, I'd say something has been fixed in 5.8. :-)