Aren't you printing three separate references to the same value, rather than three separate values? That's what your backslash is introducing. The hex number is (related to) the address of the reference created by the backslash notation, not the address of the actual thing at the end of the reference.
I was under the same impression that Perl has one instance of undef, and all SVs that refer to undef refer to it. It is singleton. There's no way to form an undef SV other than sv_undef, that I know of. I think there is also a shareable instance of 1 and 0 (sv_yes and sv_no), but the use of these is not as cut and dried. Check perlguts for more information.
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