Fields of hash are the keys, the value of which we get by referring to it:
$hash.key
Such a construction will return value associated with the key. If non-existing key is referred to, the value of key _default will be returned, if specified.
Assigning something to hash key actually adds or updates a pair key/value in the hash:
$my_hash.key[value]
For better interchangeability of hashes and tables, field fields contains reference to hash itself, see "Using hash instead of table".