I replied to this already but something seems to have gone wrong and the reply didn't make it. Basically if you have 8 columns that you need to index then need 8 indexes. No way around it. If you are only retrieving the sorted list once and then forgetting about it forever, then maintaining the indices only slows you down and it's not worth it. However if you are going to retrieve it even just a few times, then it's probably a win.

You could also try DB_File with it's DB_BTREE functionality to handle the sorting and storing of the arrays. This effectively gives you a sorted hash that persists on disk between calls to your program. You would maintain 8 of these and whenever you add a message, you would do

tie %index1, "DB_File", "index1", O_RDWR│O_CREAT, 0666, $DB_BTRE
+E
tie %index2, "DB_File", "index2", O_RDWR│O_CREAT, 0666, $DB_BTRE
+E

...

sub insert
{
  my $msg = shift;
  $index1{$msg->key1} = $msg->uid;
  $index2{$msg->key2} = $msg->uid;
...
}

my @sorted_by_index1 = @uid2msg{values %index1};
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unlike a normal hash, when you use a DB_BTREE values will give you the values back in the correct order (sort by their keys)

If you go down this route you are basically implementing your own database and you may want to look at just using DBD::SQLite which gives you a fast, direct to disk database.