$ perl t.pl
Size of a: 3
$VAR1 = [
          1,
          2,
          3
        ];

Size of b: 1
$VAR1 = [
          [
            0,
            1,
            2
          ]
        ];

Size of b[0]: 3
$VAR1 = [
          0,
          1,
          2
        ];