Man pages matching name fcgi::client::constant in section 3pm:

FCGI::Client::Constant.3pm.en	debian 10 / libfcgi-client-perl
FCGI::Client::Constant.3pm.en	debian 11 / libfcgi-client-perl
FCGI::Client::Constant.3pm.en	debian 12 / libfcgi-client-perl