XML::Smart and XML::SMART both works!

... but only as long as you are on case insensitive file systems. As soon as that changes -> BOOM. So please use the correct spelling.

I am completely lost here :-(

It's not an easy topic, mostly because many modules are buggy. But I can't do more than provide you a working example.

I'll try to give you some general advice though, most of which is already in the article I linked to above.

• Forget about UTF8Simple. Now.
• Use a not-so-buggy XML module. XML::LibXML and XML::Twig both have been recommended here multiple times, and I've used both (on very small projects) with success.
• Your non-buggy XML module will decode all strings on reading, and encode them on writing. So as long as you only deal with decoded text strings, you're done. So make sure that everything that comes from the outside into your program is also decoded. Maybe IO layers (also described in this article might help you with this.
• Use encode (or IO layers) to present your data to the user
• Use Devel::Peek to debug your code.

From the example text I conclude that you speak German. If that's the case I can recommend the #perlde channel on irc.perl.org, that's easier if you have more questions and don't exactly know how to ask them.