File Locking

Trick #1: Unlocking

        # Process A

        open DATAFILE, "+< $datafile" or die ...;
        flock DATAFILE, LOCK_EX;

        seek DATAFILE, 0, 2;
        print DATAFILE, $newdata;

        close DATAFILE;

        # do something else for awhile

Using LOCK_UN is almost always a mistake
- LOCK_UN should be considered for experts only
In many ordinary situations, the simplest solution is to close the handle
This flushes the buffer and releases the lock