Everything you suggest should work fine. Generally, a hash key is just a string, and can be anything that a string can be - it can even have null bytes in it. I don't remember right now, but it is possible that a 32-bit build of perl may be limited to strings of length 2^32-1 and therefore to hash keys of that length. A 64-bit build should in any case be able to support strings up to length 2^64-1 (if you have that much memory).

If you provide the key as a literal in your perl code, you will need to take the same care to quote it correctly as you would using it as a string.

If you provide something that isn't a string then it will be coerced to a string to make the key, the same as if you were printing it. If it was an object, that means you won't be able to get back the object reference from looking at the key; if it was a number, you won't necessarily get the identical number back, at least not to the same precision.