__PACKAGE__->tt_config(
    TEMPLATE_OPTIONS => {
        # same as the original one, plus:
        PRE_PROCESS     => 'config.tmpl',
    },
);

# config.tmpl, in case I have to define more site-wide parameters in the future
[% PROCESS menu_def.tmpl %]

# menu_def.tmpl
[% site.menu = [
    { text => 'Foo',
      link => 'foo.html', },
    { text => 'Bar',
      link => 'Bar.html', },
    ];
%]

# main_menu.tmpl obviously becomes
<ul>
[% FOREACH site.menu %]
    <li><a href="[% link %]">[% text %]</a></li>
[% END %]
</ul>