Anyway, TIMTOWTDI, even for Template-Toolkit. Here's the same demo modified to use a filter, which is like a "program" (thus, a sub) you pipe the data through.

#!/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 %]
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