Still no:

BEGIN {
    &Internals::SvREADONLY(\undef, 0);
    ${ \undef } = 42;
 &Internals::SvREADONLY(\undef, 1);
}

my $x = undef;

print( ((undef) ? 'True' : 'False'), $/);
print( ((defined undef) ? 'True' : 'False'), $/);
print( (($x) ? 'True' : 'False'), $/);
print( ((defined $x) ? 'True' : 'False'), $/);
print $x;

__OUTPUT__
True
True
False
False
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Aha. my $x = undef wasn't as simple as we thought. $x was being cleared without PL_undef actually being assigned. Swap the right side for something more complicated like $hash{'non-existant'} and you'll find that $x is now 42.

That's a pity ... I thought the undef function was supposed to return PL_undef if you passed it no parameters. At least, that's what the Perl cookbook implies on p369, as well as the Camel in its discussion of the undef function in the Appendices. Why wouldn't it work? (And, yes, I'm scared of spelunking the Perl codebase.)

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