From perldoc -f system:

You can check all the failure possibilities by inspecting $? like this:

                   if ($? == -1) {
                       print "failed to execute: $!\n";
                   }
                   elsif ($? & 127) {
                       printf "child died with signal %d, %s coredump\
+n",
                           ($? & 127),  ($? & 128) ? 'with' : 'without
+';
                   }
                   else {
                       printf "child exited with value %d\n", $? >> 8;
                   }
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Update: In the case of waitpid, this $? == -1 means that the child has been reaped automatically. I encountered this some time ago in a similar snippet. I do not know exactly why but a similar effect can be obtained by:

I cannot find the program I was working on but if I'm not mistaken, I got rid of all negative ones by simplifying the SIGCHILD handler. Store $? in the hash instead of testing it and test everything when all children are done.