In the writing routine:

    flock $FH, LOCK_EX or die "Flock failed: $!\n";
    seek  $FH, 0, 2 or die "Seek failed: $!\n";
    print $FH "$address\n";
    flock $FH, LOCK_UN or die "unFlock failed: $!\n";
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at the point you release the lock the printed address will usually still be in the output buffer in memory, not written to disk.

See 'perldoc -q flush' for various ways to ensure that the output is flushed to disk before you release the lock.

Hugo