In case you haven't done a super-search for "flock", you can check this thread, which has relevant info in some of the later replies, as well as a link to a useful article from the Feb.2002 Perl Journal (Issue #23).

In short, flock is not available on Windows 9x systems, but is available on NT, XP, 2000. However, the TPJ article contains some code that you can use to create semaphore files, which can serve to accomplish the same thing as flock.