Hmm, but open has nothing to do with Apache2::Filter. I was able to append those values without problem.
Btw, I'm using now Compress::Zlib to uncompress the buffer, manipulate it, then compress it again. Working just fine.
To check encoding I'm using now

$r->headers_out->{"Content-Encoding"};
[download]

I'm very happy for finding out the solution myself after 2 days of tests :)