getsockopt(2) - perldoc - phpman

    getsockopt SOCKET,LEVEL,OPTNAME
            Queries the option named OPTNAME associated with SOCKET at a
            given LEVEL. Options may exist at multiple protocol levels
            depending on the socket type, but at least the uppermost socket
            level SOL_SOCKET (defined in the "Socket" module) will exist. To
            query options at another level the protocol number of the
            appropriate protocol controlling the option should be supplied.
            For example, to indicate that an option is to be interpreted by
            the TCP protocol, LEVEL should be set to the protocol number of
            TCP, which you can get using "getprotobyname".

            The function returns a packed string representing the requested
            socket option, or "undef" on error, with the reason for the
            error placed in $!. Just what is in the packed string depends on
            LEVEL and OPTNAME; consult getsockopt(2) for details. A common
            case is that the option is an integer, in which case the result
            is a packed integer, which you can decode using "unpack" with
            the "i" (or "I") format.

            Here's an example to test whether Nagle's algorithm is enabled
            on a socket:

                use Socket qw(:all);

                defined(my $tcp = getprotobyname("tcp"))
                    or die "Could not determine the protocol number for tcp";
                # my $tcp = IPPROTO_TCP; # Alternative
                my $packed = getsockopt($socket, $tcp, TCP_NODELAY)
                    or die "getsockopt TCP_NODELAY: $!";
                my $nodelay = unpack("I", $packed);
                print "Nagle's algorithm is turned ",
                       $nodelay ? "off\n" : "on\n";

            Portability issues: "getsockopt" in perlport.