in reply to Proper use of flock
DO NOT unlock the file, ever. Just close it. If you want to read from a file and then also write to it, then open it for reading and writing once, and get a shared lock. Once you want to write to it, upgrade to an exclusive lock by calling flock on the file handle again - do not close and reopen it. Don't downgrade that lock to a shared one either.
See perldoc -f flock for help on the function.
Makeshifts last the longest.