my $dbh = DBI::connect(@connect_values_here);
get_categories_at( undef );


sub get_categories_at {
  my $parent_id = shift;

  my @values;
  my $sql = "SELECT * FROM TABLE WHERE ";

  if (not defined $parent_id) {
     $sql .= "parent_id IS NULL";
  }
  else {
     $sql .= "parent_id = ?";
     push @values, $parent_id;
  }

  my $hash_ref = $dbh->selectall_hashref( $sql, 'id', {}, @values);

  for my $id (keys %{$hash_ref}) {
    print "ID => $id\n";
    get_categories_at( $hash_ref->{$id}{parent_id} );
  }
}