0> my $x = {a => 'b', c => 'd', undef => 'f'}
$res[0] = {
'a' => 'b',
'c' => 'd',
'undef' => 'f'
}
1> $x->{undef}
$res[1] = 'f'
2> my $undef = undef
$res[2] = undef
3> $x->{$undef}
Use of uninitialized value $undef in hash element at reply input line 1.
$res[3] = undef