#!/usr/bin/perl -w

use strict;
use warnings;
use Template;

my $tt = Template->new(FILTERS => { emphasise => \&emphasise });

my $data = { word => 'callbacks' };

$tt->process(\*DATA, $data) || die $tt->error(), "\n";

sub emphasise {
  return "** @_ **";
}

__DATA__
This is a template.
It includes [% word | emphasise %]