Passing the file-handle-to-be-locked to children avoids one race condition and allows the parent to unlink the temporary lock file as soon as it is created (improving security).

Unfortuantely, BSD-based flock has different ideas about such things, allowing different processes to share the lock (IIRC). This makes some things easier and some things harder. Perhaps letting the children inherit the handle-to-be-locked but having each child dup() it (see open) instead of locking it directly would give you the best of both worlds (I think and hope that BSD doesn't go so far as to prevent that from working).

Since it is output from Perl that is trying to be serialized (in the sense of synchronization not in the sense of marshaling), one likely bug would be:

    lock(...);
    print ...;
    unlock(...);
[download]

because the output doesn't happen when print is called but when the buffer is flushed. Simply adding $|= 1; should fix that in this simple case. For more complex cases, it might be easier to do:

    lock(...);
    print PIPE ...;
    close PIPE;
    unlock(...);
[download]