I don't have a win32 setup to test your script on, but Tk might be getting "bogged down" trying to update too quickly. Where you have the

       print "Got $data\n";
        .......
        ........
       $chart->redraw;
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The $chart object may be receiving redraw commands too fast. You can test it by putting a counter in there, and only redraw the chart on every 10th or 100th data input. But what I would do, is get rid of chart, and put a meter on a canvas, which will be quicker to respond to changes. See Simple Tk Gauge