Как объединить хэши, представленные ссылками?

$ perl -e 'use Data::Dumper; $a = {a=>1,b=>2}; $b = {c=>3,d=>4}; $c = {(%$a, %$b)}; print Dumper { c => $c };'
$VAR1 = {
          'c' => {
                   'c' => 3,
                   'a' => 1,
                   'b' => 2,
                   'd' => 4
                 }
        };

$ perl -e 'use Data::Dumper; $a = {a=>1,b=>2}; $b = {c=>3,d=>4}; $a = {(%$a, %$b)}; print Dumper { a => $a };'
$VAR1 = {
          'a' => {
                   'c' => 3,
                   'a' => 1,
                   'b' => 2,
                   'd' => 4
                 }
        };

perl -e '$h1={a=>1,b=>2}; $h2={c=>3,d=>4}; $h1={%$h1,%$h2}; print join("\n", map { $_."=>".$h1->{$_} } sort keys %$h1)'

@{$h1}{keys $h2} = values $h2;

sub hash_merge{
        map {ref $_ eq 'HASH' or return warn "This is not hashref!"} @_;
        my ($h1,$h2)=@_;
        local %_=%{$h1};
        @_{keys %$h2} = values %$h2;
        #say Dumper \%_;
        while(my($k,$v)=each %$h1){
                if(ref $v eq 'HASH'){
                        $_{$k}=&hash_merge($v,$h2->{$k}) if exists $h2->{$k};
                }
        }
        return \%_;
}

@{$h1}{keys %{$h2}} = values %{$h2}