use strict;
use warnings;

use Test::More tests => 1;

my $data = [
  {a => 1, b => 2},
  {a => 3, b => 4},
  {a => 5, b => 6}
];

my $want = {
    a => [1, 3, 5],
    b => [2, 4, 6]
};

my %columns;

for my $row (@$data) {
    for my $k (keys %$row) {
        $columns{$k} //= [];
        push @{$columns{$k}}, $row->{$k};
    }
}

is_deeply \%columns, $want;