For intrepid programmers, the classic textbook Unix Network Programming by Richard Stevens (published by Addison-Wesley). Note that most books on networking address networking from the perspective of a C programmer; translation to Perl is left as an exercise for the reader.
The IO::Socket(3) manpage describes the object library, and the
Socket(3) manpage describes the low-level interface to
sockets. Besides the obvious functions in the perlfunc manpage, you should also check out the modules file at your nearest
CPAN site. (See the perlmodlib manpage or best yet, the Perl
FAQ for a description of what
CPAN is and where to get it.)
Section 5 of the modules file is devoted to ``Networking, Device Control (modems), and Interprocess Communication'', and contains numerous unbundled modules numerous networking modules, Chat and Expect operations, CGI programming, DCE, FTP, IPC, NNTP, Proxy, Ptty, RPC, SNMP, SMTP, Telnet, Threads, and ToolTalk--just to name a few.