Exercise: improve the csv reader

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Instead of an array create a hash in which the key is
the fname and the value is the hash reference.

To get the phone of 'Foo' we could write:

$data{Foo}{phone}

• First you can assume fname values are unique.
• Then improve it by checking if the fname is indeed unique and warn if not.
• Further improve it by removing the fname from the internal hashes.
• Further improve by passing the name of the key field (currently fname) as a parameter.
• Bonus: try to deal with the case when there are duplicate values for the key field without just giving an error message.