Problem with serial port access in Tk app?

 From the Cookbook item under SerialPorts I have the following code 
snippet that works in a small test program (pretty much the Cookbook item:

     # Open a serial port, and tie it to a file handle for POE.

     my $handle = gensym();
     my $port = tie( *$handle, "Device::SerialPort", "/dev/ttyS0" );
     die "can't open port: $!" unless $port;

     $port->datatype('raw');
     $port->baudrate(9600);
     $port->databits(8);
     $port->parity("none");
     $port->stopbits(2);
     $port->handshake("xof");
     $port->write_settings();

     # Start interacting with the scanner.

     $heap->{port}       = $port;

     print"DEBUG: Before ReadWrite->new\n";

     $heap->{port_wheel} = POE::Wheel::ReadWrite->new(
         Handle => $handle,
         Filter => POE::Filter::Line->new(
             InputLiteral  => "\x0D\x0A",    # Received line endings.
             OutputLiteral => "\x0D",        # Sent line endings.
         ),
         InputEvent => "got_port",
         ErrorEvent => "got_error",
						     );

     print"DEBUG: After ReadWrite->new\n";

However, when I incorporate this in a Tk based application I get the 
following output/error:

DEBUG: Before ReadWrite->new
Handle not opened for input at 
/usr/lib/perl5/vendor_perl/5.8.8/i686-linux/Tk/Event/IO.pm line 122.

In the that file, line 122 is the first code line in this snippet:

    # set the handler
    my $h = $obj->handler($imode,$cb);
    undef $obj;  # Prevent warnings about untie with ref to object
    unless ($h)
     {
      untie *$file;
     }


Any ideas?

Thanks,

John