#!/usr/bin/perl                                                                                

use strict;
use warnings;
use Template;                                                                                

my $tt = Template->new;                                                                                

my $data = { word => 'callbacks',
             emphasise => \&emphasise };
                                                                                
$tt->process(\*DATA, $data) || die $tt->error(), "\n";
                                                                                
sub emphasise {
  return "** @_ **";
}
                                                                                
__DATA__
This is a template.
It includes [% emphasise(word) %]