...
use IO::Socket::SSL;
...
print IO::Socket::SSL->new(
   PeerHost => "203.0.113.46:8883",
   SSL_verify_mode => 0,
)->get_fingerprint,"\n";
...

##</code><code>##

...
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',});
...

##</code><code>##

openssl x509 -fingerprint -sha256 \
    -in /etc/mosquitto/certs/server.crt \
| sed -n -e '/Fingerprint/ { s/ .*=/$/; s/://g; p; }'