### assume that some keys could be missing or others added

my $data2 = [
  {a => 1, },
  {a => 1, b => 2},
  {a => 3, b => 3, c=>7},
  {a => 5, b => 6, c=>8},
  {a => 5, c=>8, d=>9},
];

my %key_universe_h;
my $default =0;

# determine all possible keys (number of columns)

foreach my $href (@$data2)
{
    $key_universe_h{$_}++ foreach keys %$href;
}
my @key_universe_a = sort (keys %key_universe_h);

# proceed as in first example, but with default
# for non-existant key values

my %required2;
foreach my $href (@$data2)
{
   foreach my $column (@key_universe_a)
   {
       my $value = $default;
       $value = $href->{$column} if exists $href->{$column};
       push @{$required2{$column}}, $value;
   }
}
pp $data2;
pp \%required2;