print Dumper $_ for @{ increment(increment(\@a)) };
# Output:
[
  [
    1,
    2
  ],
  [
    2,
    3
  ]
]
[
  [
    2,
    3
  ],
  [
    3,
    4
  ]
]
[
  [
    3,
    4
  ],
  [
    4,
    6
  ]
]