Accessing serial ports under Windows

[Odud]

Has anyone experience of using the serial ports under Windows? I'm trying to send commands to the modem and read the response. I thought that I'd read all the relevant bits in the open tutorial and I have the following code

use warnings;
use Fcntl;

my $tty = "COM3";
my $text = shift || "ATH0";

sysopen(COM,$tty,O_RDWR) || die "Can't open $tty $^E";

transmit("$text");
receive();

sub transmit {
    my $data = shift;
    $data .= "\r";
    @ascii = unpack("C*",$data);
    print "transmit: [@ascii]\n";
    syswrite(COM,$data);
}

sub receive {
    sysread(COM,$buf,4);
    @ascii = unpack("C*",$buf);
    print "receive: [@ascii]\n";
    return $buf;
}

However when I run the script it hangs on the sysread instead of returning O.K. as expected. I think the port is opening correctly, because when something else is using the port the open fails with an "Access denied" message.