Here's how the SVs are created: (slimmed down version)

SV * newSVObj(void *object, char * classname)
{
    SV * result;

    if (!object)
        return newSVsv(&PL_sv_undef);
    
    HV * h = newHV();
    hv_store(h, "ETK", 3, newSViv((long)object), 0);
    result = newRV((SV*)h);
    sv_bless(result, gv_stashpv(classname, FALSE));
    SvREFCNT_dec(h);

    return result;
}
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As you can see, the SV is a hashref storing only a pointer to the original object, and through the class I keep track of the type of the object, so the typemap calls functions like:

SV * newSVEtkButtonPtr(Etk_Button *o) { 
         return newSVObj(o, "Etk::Button"); 
}
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And when these SVs are passed around as data, the get wrapped into a new SV that is made mortal, for example, the code passing data to the callback function: (cbd being a certain struct that hold relevant information to the callback)

   PUSHMARK(SP) ;
   XPUSHs(sv_2mortal(newSVsv(cbd->perl_object))); // self
   XPUSHs(sv_2mortal(newSVsv(cbd->perl_data)));   // data
   PUTBACK ;
      
   call_sv(cbd->perl_callback, G_DISCARD);
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