Receives a message on a socket. Attempts to receive LENGTH bytes of data into variable SCALAR from the specified SOCKET filehandle. Actually does a C recvfrom(), so that it can return the address of the sender. Returns the undefined value if there's an error. SCALAR will be grown or shrunk to the length actually read. Takes the same flags as the system call of the same name. See "UDP: Message Passing" in perlipc for examples.