On most UNIX platforms, the CHLD (sometimes also known as
       CLD) signal has special behavior with respect to a value
       of 'IGNORE'.  Setting $SIG{CHLD} to 'IGNORE' on such a
       platform has the effect of not creating zombie processes
       when the parent process fails to wait() on its child
       processes (i.e. child processes are automatically reaped).
       Calling wait() with $SIG{CHLD} set to 'IGNORE' usually
       returns -1 on such platforms.