The hash I'm referencing is a query to a MySQL database.

No, the error means you don't have a reference to a hash.

Despite this, the error still reports those hash keys as strings.

That's not what the error is reporting. For starters, hash keys are necessarily strings.

is there a way to ensure that when the hash is built they are of type 'HASH' and not strings?

The question makes no sense. If a hash is built, it's not a string.