Mistyping hash keys is something that happens when you use hashes. It's not something that only happens when you use hashes of hashes, nor do hashes of hashes make it worse.

And you can prevent this from happening by making all your keys constants, and prepending a + in front of each key that otherwise might be autoquoted.

Disallowing hashes of hashes sounds as useful to me as disallowing arithmetic which involves divisions, as some people don't master division.

Abigail