Win32::Commport, SerialPort stopbits woes

I'm aiming to get my PocketPC to talk to a Dallas one-wire network.
However, I'm failing at the first hurdle: getting serial
communications working under PerlCE. I don't seem to be able to change
the number of stop bits.

I have Rainer's 5.6 build of Perl with Win32::API, with Nick Peskett's
Win32::Commport and Serialport (from this site) and various other
ActiveState 5.6 libraries as required. It's running on a 3870 Ipaq,
PocketPC 2002.

The serial port opens fine, but when I try:

$PortObj->stopbits(1);

I get "Could not set stopbits on COM1:". Turning $Babble on in
Commport.pm tells me that the $CP_SettableParams = 0x7f (so stop bits
should be settable) but $CP_SettableStopParity = 0x700 (so it does not
appear that the device supports any stop bits settings [other than
zero, which is what it starts up with]).

Anyone seen this before? Any suggestions?
Ta
Ben