After looking more closely on your example (it works for me with libwww-6.04 btw) I see that you're using libwww-perl-5.805. According to changelog libwww supports Mozilla::CA since version 6.00. Also older version uses Crypt::SSLeay for https support, so my previous link to LWP::Protocol::https was misleading. In order to specify CA certificate for Crypt::SSLeay you should set $ENV{HTTPS_CA_FILE}