where can i learn more about Perl hashing (and i don't mean how to hash some data [...])

Then don't use the term "hashing". You want to learn more about the implementation of Perl hashes, not "Perl hashing". The simple answer is that they're an implementation of a hash table. More in-depth answers have already been given.

what is the complexity of retrieving hash data

The average complexity for fetching, inserting and deleting is O(1).