$VAR1 = {
          'key' => {
                          'data' => [
                                      [
                                        'Element1',
                                        'Element2',
                                        'Element3'
                                      ],
                                      [
                                        'Element1',
                                        'Element2',
                                        'Element3'
                                      ]
                                    ]
                        }

##</code><code>##

for my $elmt (@{$book->{$key}{data}}) {
    ....
}