Besides the fact that this does not work (it will if you localise @result with my)..... As they say, any recursive solution can be written as in iterative one. Taking advantage of the fact you can push onto a list you are iterating over:

sub flatten_recursive {
  my $tree = shift;
  my @result;
  for my $node (@$tree) {
    push @result, ref $node ? flatten_recursive($node) : $node;
  }
  @result;
}

print join ' ', flatten_recursive($data_struct), "\n";

sub flatten_iterative {
    my @tree = ($_[0]);
    my @results;
    for my $node (@tree) {
        (ref $node) ? push @tree, @$node : 
                      push @results, $node;
    }
  return \@results;
}

print join ' ', @{flatten_iterative($data_struct)}, "\n";
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