Thanks, that clears up most things.
I did know how post-increment works on numerical scalars, but this sort of use is new to me, and the perlop page says nothing about adding new records to a hash with ++... But this is what it seems to do.

Your code works a treat, and the memory use doesn't seem to be too bad. Thanks.

In reply to Re^4: Filtering very large files using Tie::File by elef
in thread Filtering very large files using Tie::File by elef

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