If your subclasses are already loaded, then you can use Class::CanBeA to identify which ones are subclasses of a given class.

Simply looking at filenames in @INC ain't gonna do the job right in general. For example, it might tell you that Class::DBI::ClassGenerator is a subclass of Class::DBI when it isn't - it even contains the test "use base 'Class::DBI'" just to confuse matters.