Very interesting. Now /me understands. In fact this is a question of how my works rather than how $_ while works (I confess I am a very recent convert to use strict, so this is key for me to get to grips with). I understand both better now, and I must say, at the risk of sounding like a broken record, how inspired I am by how this thread (and many others) worked. It's not just a matter of having the right answers presented clearly, but of reading them in the context of a live problem. These are lessons I shan't forget - I hope it was educational for others too. Muchas gracias to all who posted.

§ George Sherston