I don't know exactly what you are trying to address (sounds like an XY Problem to me). You can use any string as a key in a hash, as long as it is unique in the hash, but building a hash whose keys are the concatenated values of other keys sounds like a design problem to me, maybe other data structures should be used instead

Could you please give a step backwards and elaborate a bit more the kind of problem you are trying to solve and why do you think that such a hash is a good option?