my $tpl = HTML::Template->new();
my $q   = CGI->new();
$tpl->param( 'file' => $q->param('file') );

# Then in your template

<tmpl_include name="<tmpl_var name=file>">

##</code><code>##

<tmpl_if name="foo">
<tmpl_include name="foo.inc">
</tmpl_if>

<tmpl_if name="bar">
<tmpl_include name="bar.inc">
</tmpl_if>