Accept password prompt 'Enter password: '
conn mhe/&password@some_database
Set serverout on
Set Verify off
Accept some_variable prompt 'Enter a variable: '
Declare
  function get_length(p_text in varchar2)  return number 
  is
  begin
    return length(p_text);
  end;
Begin
  dbms_output.put_line('The variable entered was &some_variable with length '||
                        get_length('&some_variable'));
end;
/
Set verify on

SQL> @c:t1test
Enter password: mhe
Connected.
Enter a variable: some stupid text
The variable entered was some stupid text with length 16

PL/SQL procedure successfully completed.