I'm wondering about your example.
Your code snipit:

<part name="foo">
   <part name="bar"/>
   <part name="bar2"/>
</part>
[download]

is not valid xml. I know its psuedo-code, but it makes the sol'n confusing. Perhaps you meant:

<section name="foo">
   <subsection name="bar">
   <subsection name="baz">
</section>
[download]

Or something like that? Then you want

my $foo = new section( name => "foo" ;
$foo->addSubsection( "bar" );
$foo->addSubsection( "baz" );
$foo->startSystem;
[download]

???