use Fcntl;

sub add {
    my($file, $domain_string)=@_;
    my $tempfile="Clever_but_constant_name_here";
    my $in, $out;

    sysopen $out, "$tempfile", O_WRONLY|O_CREAT|O_EXCL or die "Temporary file $tempfile exists";
    open $in, "<$file" or die $!;
    local $/ = "\;\n\n";
    my @array = <$in>;
    close $in;
    push @array, $domain_string;
    @array = sort @array;
    print $out @array;
    close $out;
    rename $tempfile, $file or die $!;
 }