...
my $flat_data =  
   [{data=>'A'} , {data=>'B'}, {data=>'C'}, {data=>'D'},{data=>'E'}];
my $columnized_data = columnize($flat_data, 3, "across");
my $tmpl_file = ...;
my $tmpl =  HTML::Template->new(
        filename          => $tmpl_file,
        loop_context_vars => 1
    ) or die "Creation of template object from $tmpl_file failed.";
$tmpl->param( {DATA_LOOP => $columnized_data});
print "Content-type: text/html\n\n", $tmpl->output();


##</code><code>##

          A B C
          D E

##</code><code>##

   [
          {
            'COLUMN_LOOP' => [
                               {'data' => 'A'},
                               {'data' => 'B'},
                               {'data' => 'C'}
                             ]
          },
          {
            'COLUMN_LOOP' => [
                               {'data' => 'D'},
                               {'data' => 'E'}
                             ]
          }
        ];

##</code><code>##

          A C E
          B D

##</code><code>##

   [
          {
            'COLUMN_LOOP' => [
                               {'data' => 'A'},
                               {'data' => 'C'},
                               {'data' => 'E'}
                             ]
          },
          {
            'COLUMN_LOOP' => [
                               {'data' => 'B'},
                               {'data' => 'D'}
                             ]
          }
        ];


##</code><code>##

So if your existing TMPL_LOOP section looks like:

<tmpl_loop name="DATA_LOOP">
   <tr>
      <td>
         <tmpl_var name="data">">
      </td>
   </tr>
</tmpl_loop>

Change it to:

<tmpl_loop name="DATA_LOOP">
   <tr>
   <tmpl_loop name="COLUMN_LOOP">
      <td>
         <tmpl_var name="data">">
      </td>
   </tmpl_loop>
   </tr>
</tmpl_loop>

____

##</code><code>##

#!/usr/bin/perl -w
use strict;
use Carp;
sub columnize {
   my $rows_ref = shift;
   my $num_cols = shift;
   my $order = shift;
   if (!defined $rows_ref or !defined $num_cols or !defined $order) {
      croak("Undefined params not permitted:\n");
   }
   if ($num_cols < 1) {croak "Bad column-number param: $num_cols\n";}
   if ($order !~ /^(across|down)$/i) {croak "Bad order param: $order\n";}

   use integer; # use integer division for all division operations

   my $num_rows = ($#$rows_ref / $num_cols) + 1;
   my @result_rows = ();
   for (my $row = 0; $row < $num_rows; $row++) {
      for (my $col = 0; $col < $num_cols; $col++) {
         my $index;
         if ($order eq "across") {
            $index = ($row * $num_cols) + $col;
         } else {
            $index = ($col * $num_rows) + $row;
         }
         # Skip any empty spots in the last row (if "across")
         # or in the last column (if "down"):
         next if ($index > $#$rows_ref);

         $result_rows[$row]->{COLUMN_LOOP}->[$col] = $rows_ref->[$index];
      }
   }
   return \@result_rows;
}
____

##</code><code>##

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

A literal interpretation would produce:

      A D F H J   
      B E G I K
      C 

##</code><code>##

      A D G J
      B E H K
      C F I