$SIG{ALRM} = sub { die 'Timeout'; };
alarm( some decent duration );
eval {
   fork the child here
   wait for the child (waitpid() or similar)
   if the child returns reset the alarm (alarm(0))
}; 
if($@) {
    if($@ =~ /Timeout/) {
         # child exceeded authorized run time, so kill it
         kill(15, $child);
    } else {
         handle other fatal problems....
    }
}