I'm now thinking, is it possible to integrate Python, Perl and C/C++ and also doing a GUI application with this very nice mix of languages?
Anything is "possible", but whether it is necessary or beneficial is debatable and highly depends on your requirements. Don't mix if you don't need to. Use the language that best fits the domain or target requirements. 
I can't think of a scenario where one needs to mix Python and Perl as their domain is largely the same.
Using C/C++ can be beneficial in cases where you need hardcore system integration or specialized machine dependent services. Or when you need to extend Python or Perl itself (both are written in C/C++).
EDIT: if you want to do a GUI application, it is probably easier to choose a language that fits the OS you want your GUI to run in. I.e. something like (but not limited to) C# for Windows, Objective-C for iPhone or Mac, Qt + C++ for Linux etc.
Python, Perl And C/C++ With GUI