You could do the double indirection thing.

That is, instead of passing the SV that wraps your C++ object to the Perl code, pass an SvRV that points to that SV. Then your Perl code would have to indirect through the SvRV to get at the object

my $cppOref = shift;
return unless $$cppOref;

$$cppOref->whatever
[download]

That way, your C++ destructor can undef the SV, and any copies of the SvRV that the Perl code has retain will point to undef. It won't stop someone making a copy of the dereferenced SV: my $sneaky = $$cppOref; and falling in a heap by trying to use it later, but that's an education thing.