[Mimedefang] Why it is'n working

[Easy]

What's wrong with this filter:

sub filter ($$$$) {
    my ($sender, $ip, $hostname, $helo) = @_;
        if ($sender =~ /^<?mail at domain>?$/i) or ($sender =~
/^<?mail2 at domain>?$/i)  {
        action_add_header("Bcc", "backup at domain");
    }
    return action_accept();
}

I receive the error:
stderr: Possible unintended interpolation of @domain in string at
/usr/local/defang/etc/mail/mimedefang-filter line 13.

Algirdas
P.S. I'm new to mimedefang. Could you please give me some hints on how to
build the filter file.