At least for me the above method happily ignored all messages because Spamassassin sets the header
X-Spam-Flag:
to all messages.
The value is either YES or NO

so the correct way for me is
$self->getHeader('X-Spam-Flag', 1) eq 'YES'

The whole if:

if (
                        lc($self->getHeader('Precedence', 1)) eq 'list' ||
                        $self->getHeader('List-Id', 1) ||
                        $self->getHeader('List-Subscribe', 1) ||
                        $self->getHeader('List-Unsubscribe', 1) ||
                        $self->getHeader('List-Archive', 1) ||
                        $self->getHeader('List-Post', 1) ||
                        $self->getHeader('X-Mailing-List', 1) ||
                        $self->getHeader('Mailing-List', 1) ||
                        $self->getHeader('X-List', 1) ||
                        $self->getHeader('X-Spam-Flag', 1) eq 'YES'
                ) {
                $self->{_msg_flag_mailinglist} = 1;
        }