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my ($x, $y, $z) = ('', '', '');
($x, $y, $z) = split(',', $_);

$_ = 'a,b';
my ($x, $y, $z) = ('', '', '');
($x, $y, $z) = split(',', $_);
print(defined($z)?'defined':'undefined', "\n");
# prints undefined.

my ($x, $y, $z) = map { defined($_)?$_:'' } (split(',', $_))[0..2];