Thanks for the suggestions. My main goal is to just log the output, grep different strings while the called program is running, highlight errors and other stuff. But in some situations I have to provide input. I thought that I could blame open3 because I thought that the called program whould inherit the stdout-filehandle and its properties, so I wrote a wrapper-script

use IO::Handle;
STDOUT->autoflush(1);
exec ./program_to_launch
[download]

And then I called the wrapperscript with open3. But it doesnt seem to work. I think I haven't really understand where the buffering actually takes place. I will now take a look at IO::Pty.

-Stefan