With the top:

    while (my $row = $sth->fetch()) {
        for (1..$#$row) { 
            my ($parent_id, $id) = @{$row}[$i-1, $i-0];
            last if !defined($id);
            ...
        }
    }
[download]

With the bottom:

    while (my $row = $sth->fetch()) {
        my ($parent_id, $id) = @$row;
        ...
    }
[download]