my mistake. it should be:

my @arr = (
  { 'path' => '/c/', dir => '/' },
  { 'path' => '/c/a1.mp3', 'file' => 1, 'dir' => '/c' },
  { 'path' => '/c/a2.mp3', 'file' => 1, 'dir' => '/c' },
  { 'path' => '/c/bb/aa1.mp3', 'file' => 1, 'dir' => '/c/bb' },   # th
+is is /c/bb
  { 'path' => '/c/bb/aa2.mp3', 'file' => 1, 'dir' => '/c/bb' },   # th
+is is /c/bb
  { 'path' => '/c/bb/', 'dir' => '/c' },
  { 'path' => '/c/cc/', 'dir' => '/c' },
  { 'path' => '/c/aa/', 'dir' => '/c' }
);
[download]

key "dir" is only here to make it faster to compare where which file/subfolder is located. dir is nothing else but everything from the path until last slash. path =~ s/^(.*)(\/.*)$/$1/;