Explain identifier scope, visibility and qualified names in nested PL/SQL blocks

An identifier is visible from its declaration to the END of the block it is declared in, including nested blocks. If an inner block declares its own variable of the same name, the inner one wins inside that block (the outer is 'hidden'). Label the outer block (<<outer>>) to qualify the hidden name as outer.v. The same qualification (procedure_name.parameter) resolves a collision between a parameter and a column name in embedded SQL.

<<outer>>
DECLARE
  v_x NUMBER := 1;
BEGIN
  DECLARE
    v_x NUMBER := 2;             -- hides outer.v_x
  BEGIN
    DBMS_OUTPUT.PUT_LINE('inner v_x  = ' || v_x);        -- 2
    DBMS_OUTPUT.PUT_LINE('outer.v_x = ' || outer.v_x);   -- 1
  END;
END;
/