Thanks a lot hippo for your patience and your help !!!!!!!!!

I just could solve my issue now!

It seems that the server checked some information which was sent with the previous get. In python the get and the following post were done with the same agent. In perl I used LWP::Simple for the get and LWP::UserAgent; for the POST. This I was changing now and now it works. (Or I changed something which I am not aware by doing this change. The GET sent before the POST is now for sure different, in the response I see no difference between the two versions)

But without your help I would not have manged (at least not in this short time) to get the POST done fully in the needed way.

Thanks a lot !!