You could use a tied filehandle to read the continuous incoming stream. The documentation of Win32::Serialport on CPAN has a section to cover a tied filehandle.

Another observation is that your code does -

(undef, $string_in) = $ob->read(65536);
[download]

where you discarded the input size with undef. What you should do is -

my $input_string;
{
  my ($size, $str) = $ob->read(4096); # 4k at a time
  $input_string .= $str;
  redo if ($size == 4096); # read more?
}
[download]

So the idea is to keep reading the input, and append to the input string, until there is no more input.