The most important thing to note is that you're dealing with not a plain nested data structure, but an object. You can see that from the bless( ...., "'Class::DBI::Iterator'); statement surrounding the data structure.

The last item in the bless call is the class of the object, in this case Class::DBI::Iterator. If you weren't aware of that, see the docs for that class first.

As for the strucure:

{ 'x' => y } creates a hash reference with 'x' as a key an d y as the value
[ ... ] creates an array reference
0 is a number
[] is an reference to an empty array

See also perlreftut