Try changing your waitpid test to:

do {
    $child = waitpid(-1,WNOHANG)
  } while ( $child <= 0 );
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and you might want to add a print statement in front of the "return" in your SIG handler to see if waitpid might be returning -1 for those other kids.

Update:
I completely overlooked this, but system does a fork call of its own, meaning that your SIG handler should catch those as well - so it really shouldn't be necessary to fork children who fork a single child... just catch the CHLD from the system call (?) (Nevermind, I see you're wanting to capture the PID for the resources hash)

I guess try the change in the loop and let us know what happens. That seemed to help my program (unless Linux's waitpid/wait4 is that much different than AIX (crosses fingers))