Update: The question that this is answering seems to have disappeared. So bear with me if the answer seems a bit strange.

Perl might even be using fcntl for the flock function: if it can't find a local version of flock that it can use, it will use fcntl instead. Generally, it's flock or nothing: just make sure that all your processes are using flock, and everything will be happy. lockf and fcntl can still be used, but they are heavy-duty solutions, and should only be brought out when really, really needed, and you really, really know what you are doing. The higher level flock is quite sufficient for almost any task

Also, make sure that you check the return values of 'flock', as well as 'open' and 'close'. Specifically unlocking a file is seldom necessary, as 'close' is guaranteed to unlock it for you.