For sheer perversity, thanks to http://blogs.perl.org/users/josh_ben_jore/2010/08/ponies-are-the-truth.html:

&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" }
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