Hi,
I think you would all enhancements steps.
Am just giving conditions which can be useful to you.
Use conditions in your cmod logic to fill PERIO
VBRK-VBELN = FPLA-VBELN then you can fill PERIO from table FPLA.
Even if you check with your abap team also they can suggest you about possible logic.
You can explore your data source base tables and FPLA table. if you observe tables fields you may know the fields which are common in both tables. Based on common fields you can implement logic.
Thanks