#!/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) %]
[download]

I didn't know you could do it this way. Neat.

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 %]
[download]

That looks great, thanks!

my $tmpl = new HTML::Template::Expr
    filename  => 'template.html',
    functions => {
        site_name    => \&site_name,
    };
[download]

<TMPL_VAR EXPR="site_name()">
[download]

The Petal templating system also allows you to call functions on objects, and even in a relatively nice (but still ad-hoc) fashion:

# template
<a tal:attribute="href object/link" href="test_link">Test link</a>

# the matching code:
my $object = {};
bless 'Obj', $object;

my $i = 0;
sub Obj::link {
  return sprintf "http://www.google.com?q=%s+Number", $i;
};
[download]