Thanks monks!
this is correct - the only way to combined the files was to completely re-encode each mp3 with audacity. I even tried using a MP3::Tag to strip away existing tags but it was not enough. The only way is to re-encode, then the joining works. After encoding it was just simply opening each of the files, binmode both filehandle, binmode output filehandle and writing it the new file.
thanks again.
AL