Original hash
$VAR1 = {
          'key3,' => 'value1',
          'key2,' => 'value2',
          'key4,' => 'value3',
          'key6,' => 'value4',
          'key7,' => 'value4',
          'key5,' => 'value2',
          'key1,' => 'value1'
        };
Inverted hash
$VAR1 = {
          'value4' => [
                        'key6,',
                        'key7,'
                      ],
          'value1' => [
                        'key3,',
                        'key1,'
                      ],
          'value3' => [
                        'key4,'
                      ],
          'value2' => [
                        'key2,',
                        'key5,'
                      ]
        };