Man pages matching name plack::app::proxy::backend in section 3pm:

Plack::App::Proxy::Backend.3pm.en	debian 10 / libplack-app-proxy-perl
Plack::App::Proxy::Backend.3pm.en	debian 11 / libplack-app-proxy-perl
Plack::App::Proxy::Backend.3pm.en	debian 12 / libplack-app-proxy-perl