use strict;
use HTML::Template;

my $tmpl = new HTML::Template(
                              filename => 'first.tmpl'
                              );

# generate custom page link...
my $custom_page = "second.tmpl";
# inject a custom piece of Template code
$tmpl->param(special_page_include => qq/<TMPL_INCLUDE name = "$custom_page">/);

# process custom built template..
my $tmpl_final = new  HTML::Template(scalarref => \$tmpl->output());
print $tmpl_final->output();