It's really just a matter of inverting the order of the indexes.

From an earlier thread (How to swap rows with columns?),

my @data;
while (<>) {
   my @fields = split ' ';
   my $col = $. - 1;
   for my $row (0..$#fields) {
      $data[$row][$col] = $fields[$row];
   }
}
[download]

Tested.