my %hash1 = (a=>4,b=>5,c=>6,d=>7);
my %hash2 = (a=>4,b=>5,c=>6,e=>7);
{
    delete local @hash1{keys %hash2};
    print "hash1 only: $_\n" for keys %hash1;
}
{
    delete local @hash2{keys %hash1};
    print "hash2 only: $_\n" for keys %hash2;
}
# %hash1 and %hash2 are unchanged here

__END__

hash1 only: d
hash2 only: e