Assuming you're using Net::Telnet, try the 'dump_log' method as it will show the ASCII / Hex of those strange box characters. It may help determine where they're coming from if you know what they are.

$telnet->dump_log($filename);
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From 'perldoc Net::Telnet':

dump_log - log all I/O in dump format
        $fh = $obj->dump_log;

        $fh = $obj->dump_log($fh);

        $fh = $obj->dump_log($filename);

  This method starts or stops dump format logging of all the object's
  input and output. The dump format shows the blocks read and written
  in a hexadecimal and printable character format. This method is
  useful when debugging, ...
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