File::FcntlLock is an XS module precisely for that reason. Using pack isn't that easy as you make it look: you need to know the exact layout of the flock structure used on the system, and this differs considerably between system (different numbers of members, different ordering of the members, different types of members, structure padding). Thus a program creating a flock structure using pack (or diassembling it with unpack) is, unfortunately, not portable.