It would help if you could post a short, representative sample of your input, desired output, and the code you are using. See How do I post a question effectively?

Well i know generally a code example is helpfull, but i thought in this case its a pretty General question. Anyhow, I will give you some examples of what i am doing at the Moment. given is a Schema like :

-data
   -properties
     (List of properties in here)
-suites
   -tests
[download]

Up to now i got my code writing the properties.

$schema->template('PERL', $type); 
#warn $schema->template('PERL', $type);
 my $doc    = XML::LibXML::Document->new('1.0', 'UTF-8');
 my $write  = $schema->compile(WRITER => $type, use_default_namespace 
+=> 1);
 my %hash;
 #some filling of the hash with values
my $xml    = $write->($doc, $hash);
 $doc->setDocumentElement($xml);
[download]

So the first part of my XML File is generated. Later on, in a completely different part of the program, tests have to be written into the second section. Those tests have to comply the Schema in the xsd file. So i have to reopen the saved XML file from the code above and insert some new tags in it, at specific Points (every test has to go into a specific Suite, so there is more than one Suite) Writing a new hash and doing

my $xml    = $write->($doc, $hash);
 $doc->setDocumentElement($xml);
[download]

again, does not work out. I need to reopen, search a node and insert children. This was easy possible with XML Twig, but as said i can only use above mentioned packages. Was this enough code or do you Need more explaning?

General questions get general answers... there is an example of reading an XML file with XML::Compile in this article: http://perl.overmeer.net/xml-compile/papers/201208-PerlMagazine-article-en.html

That gets you a Perl data structure which you can manipulate to add the items you need. If you're not sure how to go about that, then these should be helpful: perldsc, perlreftut, perlref

An example of writing that back out is here: http://blogs.perl.org/users/brian_e_lozier/2011/10/using-xmlcompile-to-output-xsd-compliant-xml.html (note XML::LibXML::Document's toFile method is probably helpful)

That gives you everything you need to do what you describe. For more specific answers, be more specific than "does not work out", for example provide a runnable sample program that reproduces the problem. I know what I mean. Why don't you?