Can you explain to me exactly the difference between T_OBJECT and T_PTROBJ?

T_OBJECT is not a standard type. It doesn't exist in the core typemap. (I checked 5.6.1, 5.8.0, 5.8.6 and 5.8.8). If you provided me with your module-specific typemap, I'll explain what it does.

You should use T_PTROBJ or T_PTRREF, according to perlxs.

For inputs of type T_PTROBJ, the XSUB calls the XS equivalent of UNIVERSAL::isa on the argument to make sure the arguemnt is defined and a blessed reference of the appropriate class or of a descendant of the appropriate class. Outputs of this type of blessed.

For inputs of type T_PTRREF, the XSUB only checks if the argument is a reference. It doesn't care if the reference is blessed or not. Outputs of this type are not blessed.

I've verified that 123 and 'abc' are not accepted.

Only if XSRETURN_UNDEF returns immediately (as opposed to setting the return value). I don't know if it does, and you said you didn't know what XSRETURN_UNDEF does. Move RETVAL = my_c_create_bar(foo); into the "then" clause of the if to be safe.