ok, finally came around to testing this and on a debian 11 this doesn't work out of the box (or I am doing something wrong).
If I understand correctly, mailq is not provided by the package opensmtp. What actually works though, is using smtpctl show queue on debian.
Now, I don't know how common that setup is.
I guess we have to test whether PATH_TO_MAILQ is empty and search for smtpctl then, execute that and use the output.

                if ( $utils::PATH_TO_MAILQ eq "" ) {
                        # Got no Mailq
                        if (! open (MAILQ, "$sudo /usr/sbin/smtpctl show queue | ")) {
                                print "ERROR: could not open  $sudo /usr/sbin/smtpctl\n";
                                exit $ERRORS{'UNKNOWN'};
                        }

something like that, but probably less prone to breaking

@ichdasich what @RincewindsHat might want to say ... can you maybe try to implement his suggested changes? ;)