#!/usr/bin/env perl

use strict;
use warnings;

use Data::Dump;

my $data = [
    { a => 1                     },
    { a => 3,     b => 4         },
    {                            },
    {             b => 8         },
    { a => undef,         c => 9 },
];

my (%columns, %seen_key);
my @uniq_keys = grep !$seen_key{$_}++, map keys(%$_), @$data;

for my $row (@$data) {
    push @{$columns{$_}}, $row->{$_} for @uniq_keys;
}

dd \%columns;