in reply to Change package name on the fly?
ok( Foo::Bar::baz('wibble') eq Old::Foo::Bar::baz('wibble'),'baz +is compatible');
I'd just write:
is( baz('wibble'), 'wibbled' );
And then run the test suite against both versions. A test failing against the old version of the module might mean that you have to
fixchange the test (if you really mean "backward compatible" as in "bug-for-bug compatible").