$LOCK = $0;
$MAX_RUNNING = 10;

sub lock_count
{
   return scalar split /\s+/, `fuser $LOCK 2>/dev/null`;
}

die ("Too many running processes")  if (&lock_count >= $MAX_RUNNING);
open(LOCK) or die("Failed to acquire lock");
die ("Too many running processes")  if (&lock_count > $MAX_RUNNING);