There is some interesting code in "perldoc -q filehandle" on how to detect if any data is sitting in a filehandle. Search for

How can I tell whether there's a character waiting on a filehandle?

It uses ioctl and has instructions on how to create .ph files from your system .h files.

The basic idea is you can detect how much data is in a pipe and you can suck it out from the reader end.