A question was implied. Here's the relevant code, which I didn't post before because I doubt it'll help. I was hoping that someone had already encountered this trouble, which I suspect has something to do with Win32::SerialPort.

# First call this sub.

sub connectToGPS {
    my $gps;
    if ($winMain->rbNMEA->Checked()) {
        $gps = new GPS::NMEA(Port => $winMain->cbGPSPort->Text(),
                             Baud => $winMain->cbPortBaud->Text());
    }
    else {
        $gps = new GPS::Garmin(Port => $winMain->cbGPSPort->Text(),
                               Baud => $winMain->cbPortBaud->Text());
    }
    return $gps;
}

# Then pass its returned value to this sub.

sub getGPSData {
    my $gps = shift;
    my %gpsHash = %{$gps};
    my $port = $gpsHash{serial};
    $port->purge_all();
    $port->stty_kill("\cU");
    my ($time, $satsUsed, $accuracy, $quality, $speed);
    if ($winMain->rbNMEA->Checked()) {
        $gps->parse();
        my %gpsData = %{$gps->{NMEADATA}};
        $time = $gpsData{time_utc};
        $time = "---" unless $time;
        $satsUsed = $gpsData{num_sat_tracked};
        $satsUsed = "---" unless $satsUsed;
        $satsUsed = 0 if $satsUsed eq "00";
        $accuracy = $gpsData{data_valid};
        $accuracy = "---" unless $accuracy;
        $quality = $gpsData{fixq012};
        $quality = "---" unless $quality;
        $speed = $gpsData{speed_over_ground};
        $speed = "---" unless $speed;
        $speed = "---" if $speed eq "";
    }
    else {
        my ($sec, $min, $hour, $mday, $mon, $year) = $gps->get_time();
        $time = "$hour:$min:$sec";
        $time = "---" if $time eq "::";
    }
    my ($latitudeSign, $latitude, $longitudeSign, $longitude) = $gps->
+get_position();
    $port->close();

    # Irrelevant GUI update code goes here.
}
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