Implement tree: prints a tree structure of a given directory.
All filenames are printed and subdirectories are properly indented.
$ tree.pl .
.
  subdir_1
    file_1_in_subdir_1
    file_2_in_subdir_1
  subdir_2
    subdir_2_1
      file_1_in_subdir_2_1
    file_1_in_subdir_2
 
Implement the previous one using File::Find
 
Implement the previous one using File::Find::Rule