I actually found a post on comp.lang.perl.misc that was from the guy that wrote it; here is his algorithm:

enter program
while (1)
   look for lockfile
   if lockfile does not exist exit
   sleep 5 seconds
}
create lockfile

..... do normal program stuff ......

unlink lockfile
exit
[download]

However, if the process died before removing the lock, it crashed the web server. The post was 7 years ago, so no doubt what has been suggested here will be _much_ better. :)

Peter