As long as you start the child script as you are, yes, the communication directly between parent and child is limited to STDOUT and possibly STDERR. There are many ways to communicate indirectly: have the child write the xml data out to a file which the parent will open, for instance. In your case, passing the XML directly back to the parent seems the best idea.

perlipc discusses inter-process communication, exchanging messages between independent processes running at the same time. I've read that myself, but I can't claim to understand how it works. Threads are another way to accomplish the same thing and communication between is easier. See these tutorials for more information.