sub flatten {
    my @ret = ();
    while (@_) {
        my $k = shift;
        if (ref $_[0]) {
            my $v = shift;
            push @ret, [ $k, @$_ ] for flatten(@{$v});
        } else {
            push @ret, [ $k ];
#            push @ret, [ shift ] while @_ and (! $_[1] or ! ref $_[1]);
        }
    }
    @ret;
}

use Data::Dumper;
print Dumper([flatten(@{$data_struct})]);