When you open a process like that you can only read from it. That might be fine but if you need to read and write check into Expect like bruceb3 mentioned.

Update: I don't see why you couldn't do some forking and create a child process with STDIN and STDOUT pointing to file handles the parent shares and then have the child exec the cu command. Try looking in perlfaq for "dup". How do I dup a filehandle in Perl