SV * overload_add_eq(SV * obj, SV * addon, SV * third) {
     Card* c = (Card *)SvIV(SvRV(obj));
     SvREFCNT_inc(obj);
     c->value += SvIV(addon);
     return obj;
}

##</code><code>##

void overload_add_eq(SV * obj, SV * addon, SV * third) {
     Card* c = (Card *)SvIV(SvRV(obj));
     c->value += SvIV(addon);
}

##</code><code>##

use warnings;
package My_struct;

use Inline (C => Config =>
            BUILD_NOISY => 1,
            );

use Inline C => <<'EOC';

     typedef struct {
       char * suit;
       int value;
       } Card;

SV * create_struct(SV * s, SV * v) {
     Card * cptr;

     New(123, cptr, sizeof(Card), Card);
     if(cptr == NULL) croak("Failed to allocate memory in create_struct function");
     
     cptr->suit = SvPV_nolen(s);
     cptr->value = SvIV(v);
     return sv_setref_pv(newSViv(0), "My_struct", cptr);
}

void deref_ref(SV * obj) {
     Inline_Stack_Vars;

     Inline_Stack_Reset;
     Inline_Stack_Push(newSVpv(((Card *)SvIV(SvRV(obj)))->suit, 0));
     Inline_Stack_Push(newSViv(((Card *)SvIV(SvRV(obj)))->value));
     Inline_Stack_Done;
     Inline_Stack_Return(2);

}

void DESTROY(SV * obj) {
      printf("Destroying ...");
      Card* c = (Card *)SvIV(SvRV(obj));
      Safefree(c);
      printf("...destroyed\n");
}

void overload_add_eq(SV * obj, SV * addon, SV * third) {
     Card* c = (Card *)SvIV(SvRV(obj));
//     SvREFCNT_inc(obj);
     c->value += SvIV(addon);
//     return obj;
}


EOC

use overload '+='   => \&overload_add_eq;

$suit = ('Hearts', 'Spades', 'Clubs', 'Diamonds')[rand(4)];
$value = int(rand(13)) + 1;

$obj = create_struct($suit, $value);
$addon = 5;

@members = deref_ref($obj);
print "@members\n";

#$obj += $addon; # Crashes
overload_add_eq($obj, $addon, 0);

@members = deref_ref($obj);
print "@members\n";