[mp2] No data is read from Perl Output Filter

I am experimenting with ReverseProxy::FormFiller and stuck with output request rewriting. Nothing is read from filter, but filter is invoked.
Code below writes nothing into logs.
while ($f->read(my $buffer, 1024)) {
          $body .= $buffer;         
           &logErrorSimple($f->r,"---------$body------$buffer-----");
}

There are always a few parameters are sent via POST during tests, but nothing can't be rewritten by the filter because nothing is read.