$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.

In reply to Re: what is the variable type of $data = { by ikegami
in thread what is the variable type of $data = { by jimyokl

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