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

getsockname - retrieve the sockaddr for a given socket

getsockname SOCKET

Returns the packed sockaddr address of this end of the SOCKET connection.
use Socket;
$mysockaddr = getsockname(SOCK);
($port, $myaddr) = unpack_sockaddr_in($mysockaddr);