+$x # do_nothing($x->{a}); () # '', undef, { }
+$x # do_nothing($x->{a}); () # '', undef, { } #
+$x # do_nothing($x->{a}); () # '', undef, { } # +fetch
+$x # do_nothing($x->{a}); () # '', undef, { } # +exists
+$x # do_nothing($x->{a}); () # '', undef, { } # +delete
+$x # do_nothing($x->{a}); () # qr/^Can't vivify reference/, undef, undef # +store
+
+$x # set_arg($x->{a}); () # '', undef, { a => 1 }
+$x # set_arg($x->{a}); () # '', undef, { a => 1 } #
+$x # set_arg($x->{a}); () # '', undef, { a => 1 } # +fetch
+$x # set_arg($x->{a}); () # '', undef, { a => 1 } # +exists
+$x # set_arg($x->{a}); () # '', undef, { a => 1 } # +delete
+$x # set_arg($x->{a}); () # qr/^Can't vivify reference/, undef, undef # +store
+