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

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