According to perlguts, &PL_sv_undef is not the right thing to use in
those cases because it doesn't behave the same way as an undef value via
Perl code.  Seems the intuitive way to deal with undef values is subtly
enough broken that it's hard to notice when misused.

The broken uses got inadvertently introduced in commit
87bb2ade by Alexey Klyukin, Alex
Hunsaker and myself on 2011-02-17; no backpatch is necessary.

Per testing report from Greg Mullane.

Author: Alex Hunsaker