kill(0, $pid) will return true even if the
$pid process is a zombie. By setting
$SIG{CHLD}='IGNORE'; before forking, you're having the parent process reap the zombie child upon receiving SIGCHLD, causing your call to
kill(0, ...) to then work as expected.
I believe (someone correct me please) that the default perl handler for SIGCHLD is to do nothing. It is left up to the programmer to reap the child processes manually (via
waitpid, or setting
'IGNORE').
In a loop, testing the aforementioned condition, and killing the childs if the condition is met
Even after
killing the children (e.g.
kill(9, $pid), though I hope you're nicer with
kill(15, $pid)), you still need to reap them. You're doing fine since you set
'IGNORE'.