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;