open2() returns the process ID of the child process. It doesn't return on failure: it just raises an exception matching /^open2:/.
Additionally, this is very dangerous as you may block forever. It assumes it's going to talk to something like bc, both writing to it and reading from it. This is presumably safe because you "know" that commands like bc will read a line at a time and output a line at a time. Programs like sort that read their entire input stream first, however, are quite apt to cause deadlock. See open3 for an alternative.