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#!/usr/bin/env perl

...
}

dd \%columns;

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

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

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

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

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

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