Hi. I'm glad to see it's not just me that's been thinking along these lines :)

Not that I've come to any real conclusions, but I think that it's all just set theory. And the way to describe it may be those terms. i.e. sets, unions, intersections and complements.

If all the records fit in memory in a hash it's reasonably easy to describe a set that passes some test function

then you can describe the relationship you're looking for in those terms.

So you might end up with something like this:-

set1 = set of all records_1 that pass func()
set2 = set of all records_2 that fail func()
results = intersection of set1 and set2;
set3 = ...
etc
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We will needs some support functions , but that's 'just a simple matter of programming' ;)

The main problem, as I see it , is how to deal with data sets that are too big to fit into memory. My only thought is to keep a hash of the (key, file offset) and re-parse each record on demand. Or maybe Tie::File could do the job ?

Arrgh! - you're making me want to try code this again :)