Hash                         Hash Ref
my %hash = (key=>'value');   my $hash_ref = {key=>'value'};
$hash{key} = 'mod_value';    $hash_ref->{key} = 'mod_value';
print $hash{key};            print $hash_ref->{key};
my @keys = keys %hash;       my @keys = keys %{$hash_ref};