my $template = HTML::Template->new( 
    filename => 'file.tmpl',                                         
    path => [ '/path/to/templates',                                 
              '/alternate/path'                                       
            ] );

##</code><code>##

<tmpl_include name="<tmpl_var name='foo'>">

##</code><code>##

my $tmpl = HTML::Template->new( 
    'filename' => "foo.tmpl",
    'filter' => sub { 
        my $text_ref = shift;
        $$text_ref =~ s/special include tag/the actual include file tag/;
     } );