From your other questions, it seems to me as if you are confusing the keys of a hash, and the (primary) keys in a database. These two are different concepts.
If you are starting out with Perl, perlintro maybe is a good start. Also, consider the each keyword.
But thinking more about your questions, I think that you don't want a hash at all, you want to fetch (by using ->selectall_arrayref) a list of rows. These rows can be hashes again, with one key per column.
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