...
my $mqtt = Net::MQTT::Simple::SSL->new("203.0.113.46:8883", {
   SSL_ca_file   => "$cpath/certificate-authority.crt",
   SSL_cert_file => "$cpath/notifications-client.crt",
   SSL_key_file  => "$cpath/notifications-client.key",
   SSL_fingerprint => 'sha256$a...2',});
...