I don't think you can get h2xs to do anything about it, but it's easy to do manually. Just create a lib/ directory and construct your tree there. Then edit Makefile.PL and change references to Bar.pm to lib/Foo/Bar.pm. Finally, adjust MANIFEST accordingly and you're good to go.
-sam
PS: I cover this topic in my book. Buy a copy and support the artist of this message!