SQL> 
SQL> -- Forward declarations is needed in circular referencing,
SQL> --   not needed for self-referencing.
SQL> 
SQL> create or replace package test_pkg_a as
  2  function func3(p1 number) return number;
  3  end test_pkg_a ;
  4  /

Package created.

SQL> 
SQL> -- func1 and func2 are private functions
SQL> 
SQL> create or replace package body test_pkg_a as
  2  -- Forward declare private function func2
  3  function func2(p1 number) return number;
  4  
  5  function func1(p1 number) return number
  6  is
  7  begin
  8    If p1 > 20 then
  9    return p1;
 10    else
 11    return func2(p1+1);
 12    end if;
 13  end;
 14  
 15  function func2(p1 number) return number
 16  is
 17  begin
 18    If p1 > 20 then
 19    return p1;
 20    else
 21    return func1(p1+1);
 22    end if;
 23  end;
 24  
 25  function func3(p1 number) return number
 26  is
 27  begin
 28    return func1(p1);
 29  end;
 30  
 31  end;
 32  /

Package body created.

SQL> 
SQL> select test_pkg_a.func3(30) from dual;

TEST_PKG_A.FUNC3(30)                                                            
--------------------                                                            
                  30                                                            

SQL> select test_pkg_a.func3(10) from dual;

TEST_PKG_A.FUNC3(10)                                                            
--------------------                                                            
                  21                                                            

SQL> 
SQL> create or replace package body test_pkg_a as
  2  -- A self-referencing function
  3  function func3(p1 number) return number
  4  is
  5  begin
  6    if p1 < 100 then
  7    return func3(p1+10);
  8    else
  9    return p1;
 10    end if;
 11  end;
 12  
 13  end;
 14  /

Package body created.

SQL> 
SQL> select test_pkg_a.func3(10) from dual;

TEST_PKG_A.FUNC3(10)                                                            
--------------------                                                            
                 100                                                            

SQL> select test_pkg_a.func3(200) from dual;

TEST_PKG_A.FUNC3(200)                                                           
---------------------                                                           
                  200                                                           

SQL> 
SQL> drop package test_pkg_a;

Package dropped.

SQL> 
SQL> spool off