I didn't think that was obfuscatory at all, and actually I thought your first example was clearer than the second. On second glance I couldn't help but notice the split, substitute, split and thought of another way to make this a bit clearer still. Untested, I don't have the input data handy:

foreach (@in) {
  next unless m!(.*?)        # Count
              \s+
              ($in_prefix)?
              (([^/]+)/      # campaign
              ([^/]+)/       # month
              ([^/]+))!x;    # fname

  $_ = { campaign => $4,
         month => $5,
         file => $3,
         count => $1,
         sort => "$4:$5:$6" };
}
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I probably blew the regex but you get the idea. :)