my $master = HTML::Template->new(filename=>'master.tmpl');
### BLAH, BLAH ###

my $sub_tmpl = HTML::Template->new(filename=>$foo);
$sub_tmpl->param('variable'=>'value');

$master->param('include_foo' => $sub_tmpl->output);

__END__
The template like:
<tmpl_var name='include_foo' default=''>