When you are passing a list parameter the first two bytes of the parameter is a binary value that says how many list elements you sent.
For example you are sending a parm that has up to 12 library names possible that can be sent. The parm would be defined in the CL as character and 122 bytes long. Extract the first two positions of this 122 bytes using %BINARY into a decimal variable. This tells you the count of how many libraries were passed from 1 to 12. Use this counter to substring through the last 120 characters to find the library names.