recv - receive a message over a Socket
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 returns 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'' for examples.