&Internals::SvREADONLY( \undef, 0);
${ \undef } = 'true';
&Internals::SvREADONLY( \undef, 1);
my $x;
my $y = undef;
if ($x) { print "x => $x - true\n" }
if ($y) { print "y => $y - true\n" }