I don't know if it's your only problem, but you need to change push (@output, { 'row' => \@columns }); to push (@output, { 'day' => \@columns }); at least. The format you need to be returning from your sub is as follows:

return [
  {
    day => [
      { day => '' }, { day => '' }, { day => 1 },
      { day => 2 }, { day => 3 }, { day => 4 }, { day => 5 }
    ]
  },
  {
    day => [
      { day => 6 }, { day => 7 }, { day => 8 },
      { day => 9 }, { day => 10 }, { day => 11 }, { day => 12 }
    ]
  }
];
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Update: After reverse engineering the data structure you're outputting from your subroutine, I do believe that is the only change you need to make (the one s/'row'/'day'/). It should then be fine as long as the match involved with your row/columing is correct.