many thanks for Athanasius' great help!! I still want to ask question. maybe I am not good at Perl, so forgive me. According to your debug output, for the initial value for @queue and $next, I can understand them well. but after the first while loop as below:

 
(1) @queue:
$VAR1 = [
          [
            'test\\dir1',
            {}
          ],
          [
            'test\\file1',
            $VAR1->[0][1]
          ],
          [
            'test\\file2',
            $VAR1->[0][1]
          ]
        ];
--------------------------------------------------
(1) $next:
$VAR1 = [
          'test\\dir1',
          {}
        ];
[download]

why $VAR1->[0][1] is pointing to {}? I don't understand it. $ref->{'test'} or $ref->{'dir1'} seems to be "undef" for ever because the last statement is just a single $hash in "while" loop that is returned to $ref->{'test'} or $ref->{'dir1'}. for the 3rd loop as below

(3) @queue:
$VAR1 = [
          [
            'test\\file1',
            {
              'dir1' => {
                          'file3' => undef
                        }
            }
          ],
          [
            'test\\file2',
            $VAR1->[0][1]
          ]
        ];
[download]

why 'dir1' had pointed to {'file3' => undef}. $hash had never been assigned one value within in "while" loop, so $ref->{$basename} should be empty for ever when $basename was a directory. Thanks again