$ perl -e'
use Data::Dumper;
my $A = [[1,2,3],[4,5,6]];
print Dumper $A;
@$A = map { [ splice @$_, 0, 2 ], $_ } @$A;
print Dumper $A;
'
$VAR1 = [
          [
            1,
            2,
            3
          ],
          [
            4,
            5,
            6
          ]
        ];
$VAR1 = [
          [
            1,
            2
          ],
          [
            3
          ],
          [
            4,
            5
          ],
          [
            6
          ]
        ];