You don't need to dereference the hash ref into a standard hash variable to use it. As the previous poster said, this will make a copy of the hash instead of editing the original. You can use a hash ref just as you would a standard hash by dereferencing as needed. Below are comparisons of some common hash syntax.

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};
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Have a look at perlreftut for the tutorial on references. That should tell you everything you need to know :)