If I understand you correctly, it's not so much that you want to know how to initialise the hash, but what to do when a key you look for isn't there. If that's the case, you just want to do something like:

  my $val = exists $res{ someval } ? $res{ someval } : 0;
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If your "multikeys" are references to hashes, then you would assign { } instead of 0.