Sorry, I wrote a bad example, my mistake. Please look at the complete code, I am trying to build hash of hashes where each folder is a key. Early I did hash of hashes where keys are full path to folder, but it wasn't pretty.

#!/usr/bin/perl
use strict;
use warnings;
use Data::Dumper;


my %filesystem;
my $ref = $filesystem{root} ||= {};


traverse(".","");

sub traverse{
        my ($folder,@path) = @_;
        opendir DIR,$folder;
        my @list = grep {!/^\.{1,2}$/} readdir DIR; # exclude . and ..
        closedir DIR;
        foreach my $entry(@list){
                next if ( -l $folder."/". $entry); # skip links
                $ref = $$ref{$_} ||= {} for @path;
                if( -d $folder."/". $entry){
                    traverse($folder."/".$entry,@path,$entry);
                }else{
                    $$ref{$folder} = $entry;
                };
        };
};


print Dumper(\%filesystem);
[download]

if I run it it eat all my RAM, so I think I use $ref from your example wrong