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);
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, ...