Please read the section "Perl Objects And C Structures" in perlxs (ignore the part about "Ptr" getting appended). You probably want your typemap to say:

Foo T_PTROBJ
Bar T_PTROBJ
Baz T_PTROBJ
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and then have typedefs in your XS code to map these to your C level types. The perl level object hierarchy you define in the normal way in .pm files with e.g. things like:

package Bar;
...
use base qw(Foo);
...
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(and supposedly Foo.pm does the dynaload)

Again, this last is not necessarily a good idea unless it's ok to call Foo functions on Bar objects for XS methods that exist in Foo but not in Bar (they will get called through inheritance if you apply that method to a Bar object)