# config file:

engines:
   template_toolkit:
     PLUGIN_BASE: 'Template::Plugin'


# The filter code:

package Template::Plugin::FilterFence;

use Template::Plugin::Filter;
use base qw( Template::Plugin::Filter );

sub filter {
     my ($self, $text) = @_;
     return join ' ' # etc
}

1;

# The template that uses the filter:

<% USE FilterFence %>
<div id="main">
     <table>
       <% FOREACH row IN rows %>
       <tr><td><% row.0 %><td><% row.1 | $FilterFence %>
       <% END %>
     </table>
</div>