my %hash1 = (a=>4,b=>5,c=>6,d=>7);
my %hash2 = (a=>4,b=>5,c=>6,e=>7);

# make a temp. copy of the keys of %hash1
my @h1k = keys %hash1;
# delete from %hash1 all the keys in %hash2
delete @hash1{keys %hash2};
# hash1 only has keys left that are unique to it
print "hash1 only: $_\n" for keys %hash1;

# delete from %hash2 all the keys from %hash1
delete @hash2{@h1k};
# you're left with the keys that are only in %hash2
print "hash2 only: $_\n" for keys %hash2;

__END__

hash1 only: d
hash2 only: e