Man pages matching name http::proxy::engine::threaded in section 3pm:

HTTP::Proxy::Engine::Threaded.3pm.en	debian 10 / libhttp-proxy-perl
HTTP::Proxy::Engine::Threaded.3pm.en	debian 11 / libhttp-proxy-perl
HTTP::Proxy::Engine::Threaded.3pm.en	debian 12 / libhttp-proxy-perl