IDz does not support that syntax in the real-time syntax checker at this point. Support for PLI 5.2 features were added to IDz v14.1. Below is a link to the Knowledge Center: https://www.ibm.com/support/knowledgecenter/SSQ2R2_14.1.0/com.ibm.etools.getstart.wsentdev.doc/topics/whatsnew.html?cp=SS7EY3_3.1.0
Support for Enterprise PL/I for z/OS V5.2 IBM Developer for z Systems Version 14.1 provides support for Enterprise PL/I for z/OS V5.2. For more information, see Enterprise PL/I for z/OS V5.2.
The Enterprise PL/I for z/OS v5.2 mentions the use of the double slash (//) as a comment in the Language Reference guide > Delimiters and Operators. https://www.ibm.com/support/knowledgecenter/SSY2V3_5.2.0/com.ibm.ent.pl1.zos.doc/lr/delimo.html
I suggest that you open a Request for Enhancement (RFE). Below is a link to the site where you can make this request: https://www.ibm.com/developerworks/rfe/execute?use_case=submitRfe(Disclaimer: This answer is migrated from IBM Developer Answers that is no longer in use. Its original author is candice.)