For text manipulation, you can place a symbol (label?) before the DB directive:

HELLOSTR: DB 'HELLO WORLD'
HELLOSTRLEN EQU $-HELLOSTR

It may be helpful to add the second line too when it comes time to manipulate the string, for example sending it through the UART:

MOV DPTR,#HELLOSTR ; string start address
MOV R1,#0 ; string index

MOV A,R1 ; Get string char
MOVC A,@A+DPTR
MOV SBUF,A ; Send character
JNB TI,$ ; Wait for TI
INC R1 ; Next string character
CJNE R1,#STRLEN,$-8 ; Repeat until end of string

- Lee