That's not a very efficient way to perform the immediate task at hand, but it's a decent learning exercise.

The reason your program isn't exiting is that the $writer session is still active. You got an EOF on STDIN, which shut down the $reader, but $write_pipe is still open and waiting for data.

If you close $write_pipe in handle_input_error() it will cascade the EOF from $reader to $writer, at which point the $writer session will end, and the program will exit.