You should collect string in to internal table then make a loop.
DATA: lv_string TYPE string,
lt_split TYPE TABLE OF char40,
lt_char128 TYPE TABLE OF char128,
lv_wa_str TYPE string,
lv_len TYPE int4,
lr_split TYPE REF TO char40,
lr_char128 TYPE REF TO char128.
DO 50 TIMES.
CONCATENATE lv_string
`This is hello world program`
INTO lv_string.
ENDDO.
SPLIT lv_string AT space INTO TABLE lt_split.
LOOP AT lt_split REFERENCE INTO lr_split.
lv_len = STRLEN( lv_wa_str ) + STRLEN( lr_split->* ).
IF lv_len LT 128.
CONCATENATE lv_wa_str lr_split->* INTO lv_wa_str SEPARATED BY space.
ELSE.
APPEND lv_wa_str TO lt_char128.
lv_wa_str = lr_split->*.
ENDIF.
ENDLOOP.
APPEND lv_wa_str TO lt_char128.