The only concern I'd have with this technique is the problem tomhukins described earlier. I'm not sure if what he describes is likely, and whether it's more likely with a read/write filehandle than otherwise, but it's something to consider.

Using a semaphore-like technique (whether it locks the file being modified or another file) also lends itself to situations where your data processing is abstracted away from your data input and output.

Thanks for the example!

Alan