Ok, I've edited to close and open the filehandler each time the loop loops (I agree, seems kind of inefficient, that's why I kept it out of the loop at first). Maybe I'll compare that with the truncate and seek option at some point, but for now it's good enough.

I also get what you mean by using the "END" block. I never knew of it's existence and it does indeed do exactly what I want to do with my cleanup sub. Thanks for that.

The reason I had quit using die was precisely so I could call my sub. But now I have the END construct I have changed all the "unless..."s back to "or die..."s.

I'm not sure I would ever have figured out what I had not understood properly without your help. Even the page for "print" on perldoc doesn't really explain this...

You've been a great help! Thanks again.

All the best, Mark.