Category 1
    Forum 1    threads: 10   replies: 20
    Forum 2    threads: 1     replies: 0
Category 2
    Forum 3    threads: 14   replies 42

etc.

##</code><code>##

@all_categories = (
   {
      name => 'Category 1',
      forums => [ 
        {
           name => 'Forum 1',
           forum_id => 1,
           thread_count => 10,
           replies  => 20,
        },
     ...
     ],
  },
  ...
);

##</code><code>##

           SELECT 
            IFNULL(COUNT(mx_replies.forum_id),0) AS reply_count, 
            IFNULL(COUNT(mx_threads.forum_id),0) AS thread_count, 
            IFNULL(MAX(mx_threads.id), 0)  AS last_post, 
            IFNULL(mx_threads.is_new, 0) as is_new, mx_forums.id, 
            mx_forums.forum_name, mx_forums.forum_description
           FROM 
            mx_forums
           LEFT JOIN 
            mx_threads
           ON
            mx_threads.forum_id = mx_forums.id
           LEFT JOIN
             mx_replies
           ON
            mx_replies.forum_id = mx_forums.id
           WHERE
            mx_forums.category = ?

           GROUP BY 
            mx_forums.id,forum_name, forum_description
           ORDER BY 
            mx_forums.create_date DESC

##</code><code>##

    for ( @{ $categories } ) {

        $sth->execute( $_ ) or die $self->dbh->errstr;
        my $forums = $sth->fetchall_arrayref({}); 
        push @all_categories, { name => $_, forums => $forums, };
    }


##</code><code>##

$tmpl->param( categories => \@all_categories );

##</code><code>##

<ol>
<tmpl_loop categories>
<li> <tmpl_var name>
       <ol>
       <tmpl_loop forums>
       <li> <tmpl_var id> <tmpl_var forum_name>: <tmpl_var forum_description> 
              threads: <tmpl_var thread_count> last post: <tmpl_var last_post>
              replies: <tmpl_var reply_count>
      </li>
      </tmpl_loop>
      </ol>
</li>
</tmpl_loop>
</ol>