Lists don't have keys. For arrays, keys returns the indices of the elements, not every second element. So, given a list, should keys use the hash semantics or the array one?

Also note that a "list" in fact doesn't exist in Perl. There's a list context, but there always has to be something in the context, and this something is never a list. It might be a hash, it might be an array, or a sequence of comma operators. But keys doesn't want to see the hash (or array) in a list context, it wants to see its underlying HV or AV (which also makes getting the keys much faster, Perl doesn't really iterate over alternating keys and values, skipping the latter).