package MyHandler;
   sub handler($$) {
      my($class, $r) = @_;

      $class->do_this();
      $class->do_that();
   }

   sub do_this {
     # some default behavior
   }
   sub do_that {
     # some default behavior
   }

   package MyHandlerSubClass;
   use base qw(MyHandler);
   sub do_this {
      # do something else
   }
   sub do_that {
      ...
   }