I would suggest you make the hash the return value of the function:

sub unfold_hash {
    my ( $raw_hr ) = @_;
    my %res;

    for my $k ( keys %$raw_hr ) {
        $res{$k} = $raw_hr->{$k}; 
    }

    print "unfold_hash ", %res ? scalar %res : 0, "\n";
    \%res;
}

$result_hr= unfold_hash($data);
print Dumper $result_hr;
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