TMTOWTDI (couldn't figure out how to do this with just an "or" or an "and")

use strict;
use warnings;
use Data::Dumper;

my $hash;
$hash->{'a'} = 1;
$hash->{'b'} = undef;
$hash->{'c'} = undef;
$hash->{'d'} = undef;

unless (grep {defined $$hash{$_} xor $$hash{$_}=undef} keys %{$hash} )
+ { print "empty\n";}
else  { print "Not empty\n"; }

print Dumper \$hash;
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-enlil