thanks choroba and QuillMeantTen. I am still confused. what is its value of "$hash" after performing "unshift @queue , map {$_, $hash} @new_paths;" And I always feel that the value of "$hash" is "undef" all alone, because "map" seems not to be able to assign a value to $hash.

 
root@TestMachine:~/excerise_perl# perl data_for_path.pl 
$VAR1 = {
          'test' => {
                      'file2' => undef,
                      'file1' => undef,
                      'dir1' => {
                                  'file3' => undef
                                }
                    }
        };
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$hash is a hash reference. The referenced hash will get populated in a later iteration of the loop when it becomes $ref. The important thing is that the $hash returned from the do is the same empty hash as the one stored in the @queue by the map.

($q=q:Sq=~/;[c](.)(.)/;chr(-||-|5+lengthSq)`"S|oS2"`map{chr |+ord
}map{substrSq`S_+|`|}3E|-|`7**2-3:)=~y+S|`+$1,++print+eval$q,q,a,
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Hi choroba, for your words "The referenced hash will get populated in a later iteration of the loop when it becomes $ref ". can you explain/demonstrate it in detail:). I think the $hash is "undef" for ever.. Thanks a lot.