How can I change the case of a hash key?

I am writing a script which is likely to be modified by users. Currently I am storing the configuration settings inside the script. It exists in the form of a hash-of-hashes.

I would like to guard against people accidentally using lowercase characters in the hash keys, because that will break my script.

It would be simple to inspect the hash keys and merely issue warnings for any keys with lowercase characters, but I would rather fix the case sensitivity automatically.

In other words, I want to convert all the hash keys in the top-level hash to uppercase.