...
GetName(SV *thingy)
  PREINIT:
    IV tmp;
  CODE:
     if (sv_derived_from(thingy, "Foo")) {
         fooRef foo;
         tmp = SvIV((SV*) SvRV(thingy));
         foo = INT2PTR(fooRef, tmp);
         getName(foo);
     } else if  (sv_derived_from(thingy, "Bar")) {
         barRef bar;
         tmp = SvIV((SV*) SvRV(thingy));
         bar = INT2PTR(barRef, tmp);
         getName(bar);
     } else if (sv_derived_from(thingy, "Baz")) {
         bazRef baz;
         tmp = SvIV((SV*) SvRV(thingy));
         baz = INT2PTR(bazRef, tmp);
         getName(baz);
     } else croak("Some unhandled argument type message");
     ...