Some indenting is always usefull to see what happened,
I don't know the use of it, but declare :
v_weight NUMBER(3) := 1;
v_message VARCHAR2(255) := 'Product 11001';
v_new_locn VARCHAR2(50) := 'Europe';
on a higher level, or in this way:

DECLARE
v_weight NUMBER(3) := 600;
v_message VARCHAR2(255) := 'Product 11002';
BEGIN
/* SUB-BLOCK */
DECLARE
v_weight NUMBER(3) := 1;
v_message VARCHAR2(255) := 'Product 11001';
v_new_locn VARCHAR2(50) := 'Europe';
BEGIN
BEGIN
v_weight := v_weight + 1;
v_new_locn := 'Western ' || v_new_locn;
END;
v_weight := v_weight + 1;
v_message := v_message || ' is in stock';
v_new_locn := 'Western ' || v_new_locn;
END;
END;
/