When invoking new Mail::Mailer for smtp, the keyword needs to be "Server", not "server". See line 15 of Mail::Mailer::stmp.pm. So this will work:

eval {
    my $mail = new Mail::Mailer ('smtp', Server=>$SERVER);
    $mail->open({    From=>$from,
            To=>$to,
            Subject=>$subject,
        }) or die ("Can't send email headers: $!\n");
    print $mail $body;
    $mail->close();
};
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