See the current Perl documentation for recv.
Here is our local, out-dated (pre-5.6) version:

recv - receive a message over a Socket

recv SOCKET,SCALAR,LEN,FLAGS

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 for examples.