{} is an empty hash reference. I could have started with an empty hash, which might have been a little easier to understand, and I should have annotated my code. So I'll do that here:

# Read in my list of paths and cut off the newlines
my @list_of_paths = <DATA>;
chomp @list_of_paths;

my %hashed_path = ();
for my $path_string (@list_of_paths) {
  # This tracks our descent in the path tree. We start at the top
  my $hpref = \%hashed_path;
  # Split the path into its component directories, and walk through th
+em
  for my $dir (split m:/:, $path_string) {
    # At each level, if we've never been there before, create an entry
+ for it
    $hpref->{$dir} = {} unless exists $hpref->{$dir};
    # Then descend into it so we're ready to insert the next level
    $hpref = $hpref->{$dir};
  }
}

# See what the structure looks like
use Data::Dumper;
print Dumper \%hashed_path;
__DATA__
/top/middle/bottom
/top/middle/newleaf
/top/new_middle/bottom
[download]

Output is

$VAR1 = {
          '' => {
                  'top' => {
                             'middle' => {
                                           'newleaf' => {},
                                           'bottom' => {}
                                         },
                             'new_middle' => {
                                               'bottom' => {}
                                             }
                           }
                }
        };
[download]

The topmost level has only the key of empty string because that's the first value in a split on '/' when the string starts with '/'.