$data is a scalar. Its value is a reference to a hash. ({ } returns a reference to a newly-constructed hash.) Similarly, the values of the referenced hash are also reference to hashes. We call this a "Hash of Hash" or "HoH". While it's technically a reference to a hash of references to hashes, the presences of references is implied because scalars (e.g. $data, $data->{a10}) can't contain hashes directly.