There are some restrictions on the declaration of variables: : Introduction « PL SQL Programming « Oracle PL/SQL Tutorial

1. Oracle PL/SQL Tutorial
2. PL SQL Programming
3. Introduction

There is no forward declaration.

Multiple declarations are supported in PL/SQL, as shown here:

SQL> declare
  2      --v1, v2 NUMBER; -- INVALID
  3      -- VALID
  4      v1 NUMBER;
  5      v2 NUMBER;
  6  begin
  7      NULL;
  8  end;
  9  /

PL/SQL procedure successfully completed.