#!/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;

##</code><code>##

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

##</code><code>##

push @{$columns{$_}}, $row->{$_} // '' for @uniq_keys;

##</code><code>##

push @{$columns{$_}}, defined($row->{$_}) ? $row->{$_} : '' for @uniq_keys;

##</code><code>##

{
  a => [1, 3, "", "", ""],
  b => ["", 4, "", 8, ""],
  c => ["", "", "", "", 9],
}

##</code><code>##

my $sql= new SQL::Abstract( array_datatypes => 1 );

##</code><code>##

my $sql = SQL::Abstract::->new(array_datatypes => 1);