The result is something like $VAR1->{'1'}{'test2'}.
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That's just due to the way Data::Dumper has to represent the data. In order to allow for the feature to be able to recreate the original data structure by eval-ing Data::Dumper's output, it cannot repeatedly expand substructures, because in that case you'd get a new instance of the structure (hash, array,...), instead of a reference to the existing structure.

Try print Dumper $ref_2a, $ref_2b; in VERSION 2, and you'll see that it now also shows

$VAR2 = {
          '2' => {
                   'test1' => {
                                '1' => $VAR1->{'1'}{'test1'},
                                ...
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instead of the expanded style you got before

$VAR1 = {
          '2' => {
                   'test1' => {
                                '1' => {
                                         'key2' => '2',
                                         'key1' => '1',
                                         'key3' => '3'
                                       },
                                ...
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That's because now it has already encountered (and displayed (or created upon eval-ing ...)) the structure referenced by $VAR1->{'1'}{'test1'}.