It should be even simpler:

my @folder_names = ();
my %folder_contents = ();
my $folders_collection = $namespace->Folders();
foreach my $folder (in( $folders )) {
  my $folder_name = $folder->{Name};
  my $folder_items = $folder->Items();

  push @folder_names, $folder_name;
  $folder_contents{$folder_name} = [];

  foreach my $item (in( $folder_items )) {
    push @$folder_contents{$folder_name}, $item->{Name};
  }
}
[download]

dmm