I have to apologize!
The problem was not the file upload. I stored the uploaded images in a mysql database. And the error came from the image view script. Under Linux everything was okay, but under Windows the statement handle ($sth) was still active after the query (SELECT binary FROM picture WHERE id=3) and so a warning occured and was appended to the picture. That's why the image did not show correctly.

Thank you for your help!

I will have a look at CGI::Simple. Thanks.