SQL & PL/SQL :: Difference Between Variables Declared Under Begin And Declare?

i just what to understand the differnce between the variable declared under the Begin and variable declared under Declared. to understand this i tried a sample plsql procedure as show below

create or replace
PACKAGE BODY TEST_DEV AS
PROCEDURE TESTING(I_NAME VARCHAR2,
I_AGE NUMBER) AS
INTEGE binary_integer := 100;
[Code] .......

the Line L_nums1 NUMBER := 100/0; is throwing error(PLS-00103: Encountered the symbol "NUMBER" when expecting one of the following:)

if i remove the line L_nums1 NUMBER := 100/0; and just declare the variable out side the begin , and using inside the same variable inside the begin works fine

Why question is why we are not able to decalre, initialize and use the same Number variable insde the begin