For this code

    until (flock FH, LOCK_EX) {
        sleep 1;
        die "Can't get exclusive lock $!\n" if $tries++ > $timeout
    }
[download]

either change LOCK_EX to LOCK_EX|LOCK_NB or just drop the whole loop counting retries because flock(FH,LOCK_EX) simply waits until the lock is obtained.

Also, the seek isn't needed on any platform I know of. Windows needs the locking but already seeks each time you write to a file in append mode (though it certainly won't hurt). And I'd be sure to turn on "autoflush" for the file handle since you are using print to write the data.

        - tye (but my friends call me "Tye")