I assume from what you wrote that this directory is shared over the network - what's the file sharing protocol? Does it support locking files (flock or one of several CPAN modules)? If so, you could control access by having the Perl processes require a lock on a separate control file to get permission to move a file out. Otherwise, one idea might be to have some kind of simple daemon that the different Perl processes connect to which acts as a semaphore.