Wahoo! After a bit of goofing around and some pure luck I was able to make it work. I modified Win32::ComPort.pm, increasing the CP_Length from 64 to 66 as follows:
if (($CP_Length > 64) and ($self->{"_TYPE"} == PST_RS232)) {
to
if (($CP_Length > 66) and ($self->{"_TYPE"} == PST_RS232)) {
Then I installed Device::Modem and wrote the following:

#!perl
use strict;
#http://search.cpan.org/~cosimo/Device-Modem-1.51/
use Device::Modem;
my $port="COM9";
my $modem = new Device::Modem( port =>$port );
if($modem->connect( baudrate => 115200,databits=>8,parity=>'none',stop
+bits=>1 ) ) {
    print "connected!\n";
    }
else{
    print "Error: Unable to connect with $port!.\n\t$!\n";
    exit;
    }
print "CMD->";
while(1){
    my $cmd=<STDIN>;
    if($cmd=~/^(x|q|exit|quit)$/is){last;}
    $cmd =~s/[\r\n\t\s]+$//;
    $cmd =~s/^[\r\n\t\s]+//;
    $modem->atsend($cmd . Device::Modem::CR);
    print $modem->answer();
    }
#disconnect from the Modem
$modem->disconnect();
print "\nDONE\n";
[download]

And it all works. I am able to communicate to my usb device, passing it commands and seeing the returning answer. Wahoo!