Hi Prasad,
In case of modification, you need to provide the complete key for that record.
In this case, you have to pass BEGDA and ENDDA as a key value along with PERNR.
Pass the BEGDA to VAlidityStart Date and ENDDA to validity end date parameters.
Also, let me know what is the error value it shows in ls_return (in your error case )
Regards,
Venkat