Perhaps I'm missing the point of your question but.....
If the fields are ordered as you seem to suggest and the unique id is in position 1 (so you can short circuit for speed - otherwise you'd have to regex a whole line - slower than anchoring at start or substr) can't you just treat the csv files as text files, ie a bunch of arrays. Parse each one and compare position 1 (the id field) with a cumulative output array for uniqueness. So long as the output is less than about 100,000 not-huge lines Perl should do this in a few seconds per input file (if you've optimised your code).
For obvious reasons simple textfile handling is a lot faster than using CSV libraries)
I wouldn't know how to do this with a one-liner but I don't see why you would have to. Maybe it's not the neatest of solutions but it is a guaranteed, self contained solution for half hours work, that other people will easily understand in the future.
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