CGI is a text-based protocol that closely mimics HTTP/1.1 on the wire. HTTP/2 is a binary protocol designed to multiplex multiple transfers at the same time. I'm afraid that teaching CGI.pm to speak HTTP/2 is out of question. Actually, the CGI RFC defines the NPH output as an HTTP/1.0 or HTTP/1.1 response message, so unless there are servers that set SERVER_PROTOCOL=HTTP/2.0, no CGI script will speak HTTP/2, Perl or otherwise.