How to define and call an internal subroutine? | SAP-ABAP

  Subroutines are defined at the end of the program after all the events are executed. It cannot be nested inside of the events that are to be executed. Subroutines definition cannot be nested. It also supports recursion as it can call itself or a subroutine that calls it.
Syntax:
PERFORM...[USING  ...<Pi>...]
          [CHANGING...<Pi>...].