I think you are right, in such circumstance I think I would use an index:

Something like:

for my $i (0..$#{$data}) {
    foreach ( keys %{$data->[$i]} ) {
        $columns->{$_}->[$i] = $data->[$i]->{$_} ;
    } ;
}
[download]

Also makes it fairly easy to see what happens:

$columns->{$_}->[$i] = $data->[$i]->{$_} ;
hash------^-------------------------^
array-----------^-------------^
[download]