Both ways are non-destructive (after using map in second).

FWIW: I don't understand what "after using map in second" means, but the second OPed method is destructive (i.e., changes  @orig array):

c:\@Work\Perl\monks>perl -wMstrict -MData::Dump -le
"my @orig = qw(
   abcde
   fghij
   klmno
   pqrst
   uvwxy
   );
 ;;
 my @transposed;
 for (map $_ = reverse, @orig) {
   my $i = 0;
   $transposed[ $i ++ ] .= chop while $_;
   }
 dd \@transposed;
 dd \@orig;
"
["afkpu", "bglqv", "chmrw", "dinsx", "ejoty"]

["edcba", "jihgf", "onmlk", "tsrqp", "yxwvu"]
[download]

But changing the map expression to
    for (map scalar(reverse), @orig) { ... }
fixes this.