I never had to worry about using up too much memory these days because my E2900 has 32Gb of RAM, so I would always build an enormous hash in memory. 100Mb doesn't sound like too much data at all, I would build a hash in memory with the sender id as the key, then the messages in the array:

my %log = (
  'user1' => [ 'msg1',
               'msg2',
               ...
             ],
  'user2' => ...
);
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Nowadays even a half decent machines should have plenty of memory. I'd say just read into memory and write out one user at a time.