This is the simple code which will show you as to how you can call the external routine from the current program.
Program 1
REPORT ZSOURCE2307.
* List of the current program
WRITE / 'I am program ZSOURCE2307'.
* External perform
PERFORM EXTFORM IN PROGRAM ZSOURCE2308.

Program 2
REPORT ZSOURCE2308.
* Form definition
FORM EXTFORM.
WRITE / 'I am extform in program ZSOURCE2308.'.
ENDFORM.

If you like this post, you may as well like these too:

1. ABAP Programs: External flow of control (events) REPORT ZSOURCE0901. * Disp
2. EDITOR-CALL : ABAP keyword a day EDITOR-CALL Call editor fo
3. ABAP Program: Dynamic external perform (call back form) Here the two programs whic
4. ABAP Program : Working with Temporary Programs Ever had a need to create
5. Call an ABAP program from a BSP Call an ABAP program from