$ perl -MDP -wE'sub foo{DDumper[DDual($_[0])]};foo(42);foo("42");' [ undef, 42, undef, undef, 0 ] [ 42, undef, undef, undef, 0 ] $ perl -MDP -wE'sub foo{DDual($_[0]);1;};foo(42);foo("42");' IV(42) PV: SV_UNDEF IV: IV(42) NV: SV_UNDEF RV: SV_UNDEF PV("42"\0) PV: PV("42"\0) IV: SV_UNDEF NV: SV_UNDEF RV: SV_UNDEF