Thanks for all your comments.

Roger, yes, your "insane" suggestion is along the lines that I was thinking. :-) However, I fear the Windows Driver Dev Kits may be a little beyond my Perl dev skills. :-( (I will go on the hunt for software that may achieve this, however.)

The pipe suggestions would normally be ideal, but don't work for my specific circumstance. Basically, the DOS app runs continually, but periodically reads the file which I want to intercept access to. (My fault for not detailing that in my problem description!)

You have all given me food-for-thought, however, and I will renew my searching and go read up on named pipes!

Many thanks, Neil