my $sth = $dbh->prepare("SELECT category_id, category_name, main_category FROM test");
$sth->execute();
$sth->bind_columns(\my($category_id, $category_name, $main_category));
my @cats;
while ($sth->fetch) {
  $cats[$category_id][0] = $category_name;
  push @{$cats[$main_category][1]}, $category_id;
}