How do I declare/create a structure?

In general, you don't "declare" a structure. Just use a (probably anonymous) hash reference. See perlref and perldsc for details. Here's an example:

    $person = {};                   # new anonymous hash
   $person->{AGE}  = 24;           # set field AGE to 24
   $person->{NAME} = "Nat";        # set field NAME to "Nat"