my %hash = (
    'animal' => 'bear',
    'plant' => 'oak',
    'fungus' => 'truffle',
);
test(\%hash);

print join(",", keys (%hash)),"\n";

sub test {
    my ($ref) = @_;
    my %ref_hash = %{$ref};
    $ref_hash{'new'} = 'new_v';
    print join(",", keys (%ref_hash)),"\n";
}

##</code><code>##

plant,new,fungus,animal                                                                                                                                                            
plant,animal,fungus

##</code><code>##

plant,new,fungus,animal                                                                                                                                                            
plant,new,fungus,animal