Assuming that you are doing this on some sort of unix-like OS, then grepping the output of `ps ax | grep program.pl | wc -l` ought to give you the current number of running processes from that program. If 1 then it is the only instance. If 2 then another is already running, and you can exit the new instance.

There may be a better way of doing this without making calls outside perl - I still talk 'perl baby-talk' quite a lot!

Clare