This example, tested on Oracle 11, shows how you can define bind variables in SQL*Plus, assign values to them in PL/SQL then display those values afterwards back in SQL*Plus:
SQL> variable bv1 varchar2(3)
SQL> variable bv2 number
SQL> begin
2 select 'ABC' into :bv1 from dual;
3 select 123 into :bv2 from dual;
4 end;
5 /
PL/SQL procedure successfully completed.
SQL> print bv1
BV1
--------------------------------
ABC
SQL> print bv2
BV2
----------
123
SQL> execute :bv1 := 'XYZ';
PL/SQL procedure successfully completed.
SQL> execute :bv2 := 456;
PL/SQL procedure successfully completed.
SQL> print bv1
BV1
--------------------------------
XYZ
SQL> print bv2
BV2
----------
456
SQL>
