Hi, good luck on your investigation

sadly this gives me the error "Internal Server Error"... What am i doing wrong?

You're trying to replay a session for starters :) servers are free to be insane :)

Also you're using post for PUT request, use put for put request :)

Also, you're not retrieving the request you sent or tried to send from mech ... see what you tried to send to server, its Basic debugging checklist

Try HTTP::Parser::XS / Plack::HTTPParser::PP - Pure perl fallback of HTTP::Parser::XS

Along with HTTP::Body, its what Plack::Request uses

Between those modules you can build yourself a HTTP::Request you can $mech->request( $req );

Maybe See also HTTP::Request::StreamingUpload

Good luck seamonk