I see here two possibilities. First, browser requests url https://server/path, in this case it uses CONNECT method and proxy server just retransmitting data between server and browser, it doesn't analyse it in any way, it doesn't use LWP::UserAgent, so it doesn't use certificate. Second, browser requests http://server/path, in this case proxy server sends usual request to web server (not https), so it doesn't use certificate. What you should to do is to replace client request for http://server/path with request to https://server/path. I never used HTTP::Proxy but it seems it supports filters that can be used for this purpose.