Fantastic explanation! The best documentation I have been able to find about this is perlsub, but I would love to see any more details if you have them.

Here is the OP's code working as he expected:

perl -e '
> use strict;
> use warnings;
> use Data::Dumper;
>
> my $data = {status=>"ok","message-type"=>"member","message-version"=
+>"1.0.0"};
> my $result_hr;
>
> unfold_hash($data, $result_hr);
> print Dumper $result_hr;
>
> sub unfold_hash {
>     for my $k ( keys %{ $_[0] } ) {
>             $_[1]{$k} = $_[0]{$k};
>     }
>      print "unfold_hash ", $_[1] ? scalar %{ $_[1] } : 0, "\n";
> }
> '
unfold_hash 3
$VAR1 = {
          'message-version' => '1.0.0',
          'message-type' => 'member',
          'status' => 'ok'
        };
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