Is the program open or closed source, and can you tell us what it is called? Does the program's documentation say anything about using, for example, flock(2), which Perl supports? Can you contact the author(s) of the program to ask if it supports any locking or other control mechanisms? Lastly, I were you, I'd maybe try to test out if it uses some kind of locking without it being documented - again, lsof can help you find that out.