If you close the reading end of a pipe before the process writing to it has finished, then the writing process will receive a SIGPIPE, and unless it catches this signal, it will die. This dying causes close() on the reader to return false, with $? set to the signal number.

Note that pipes usually have a 4K buffer, so its possible for the writer to have finished writing and exited cleanly before the reader closes the pipe, even if all the data hasn't been read.

Dave.