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...
my $flat_data =  
...
$tmpl->param( {DATA_LOOP => $columnized_data});
print "Content-type: text/html\n\n", $tmpl->output();

          A B C
          D E

   [
          {
...
                             ]
          }
        ];

          A C E
          B D

   [
          {
...
          }
        ];

So if your existing TMPL_LOOP section looks like:

...
</tmpl_loop>

____

#!/usr/bin/perl -w
use strict;
...
   return \@result_rows;
}
____

columnize(<data set with 11 items>, 5, "down");

...
      A D F H J   
      B E G I K
      C

      A D G J
      B E H K
      C F I