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 = map {$_=>1} keys %hash1;
# delete from it all the keys present in %hash2
delete @h1k{keys %hash2};
# you're left with the keys that are only in %hash1
print "hash1 only: $_\n" for keys %h1k;

# and vice versa for %hash2:
my %h2k = map {$_=>1} keys %hash2;
delete @h2k{keys %hash1};
print "hash2 only: $_\n" for keys %h2k;

__END__

hash1 only: d
hash2 only: e