From memory of working with mainframe programmers back in the 1990s, it isn't just JCL you need. Cobol programs typically used databases and transaction monitors as well.
If you're lucky the database will be one of IBM's SQL databases. If you're unlucky it will be something like IMS.
You just reminded me that in the mid-90s I took a TCP/IP workshop where the other attendees worked on mainframes. The chasm between what I was familiar with and what they were was impressively wide.
If you're lucky the database will be one of IBM's SQL databases. If you're unlucky it will be something like IMS.