IBM In Its z/OS Communications Server Libraries does not provide an explicit PL/I Example, rather it provides a COBOL Example.
In your SEZAINST Library (TCPIP) there is the EZACICSC and EZACICSS.
Also in the z/OS Communications Server Bookshelf library there is the z/OS CS IP CICS Sockets Guide which is the documentation on using the Sockets API in High Level Languages.
In there it talks about the COBOL examples (As I mentioned above) ALONG with PL/I, Assembler, etc..
By looking at the COBOL and reading the documentation on the PL/I call formats - it is quite easy to write code to it.. I have done it many times - but Assembler for me.
Again, I know of no "explicit" code example for PL/I thats provided by IBM - only COBOL, with documentation as to the call formats dependent upon the language.
If I were you, I would start reading the z/OS Communications Server IP CICS Sockets Guide - for thats where ALL the information is about writing native CICS Sockets code for the z/OS TCPIP Stack within CICS.
Very simple. Also - dependent upon what you are trying to do, you may want to look into CICS Web Support - which is different from z/OS Communications Server IP CICS Sockets.