unshift @queue, map { [$_, $hash] } @new_paths;

##</code><code>##

while (my $next = shift @queue) {
    my ($path, $ref) = @$next;
    ...

##</code><code>##

$ref->{$basename} = do { ... };

##</code><code>##

use strict;
use warnings;
use diagnostics;
use File::Basename;                         # basename
use File::Spec::Functions;                  # catdir
use Data::Dumper;                           # Dumper
use Data::Dump;                             # dd
use constant DD => 1;                       # 0 = none, 1 = Dumper, 2 = Dump

my $data = data_for_path('./test');         # '/root/excerise_perl/test'

print "RESULT:\n";
DD == 2 ? dd $data : print Dumper($data);   # default to Data::Dumper

sub data_for_path
{
    my ($path) = @_;
    my  $data  = {};
    my  @queue = ( [ $path, $data ] );
    my  $count = 0;

    show_queue($count, \@queue) if DD;

    while (my $next = shift @queue)
    {
        if (DD)
        {
            print "($count) \$next:\n";
            DD == 2 ? dd $next : print Dumper($next);
            print '=' x 50, "\n";
        }

        my ($path, $ref) = @$next;
        my  $basename    = basename($path);

        $ref->{$basename} = do
        {
            if (-f $path or -l $path)   # plain file or symbolic link
            {
                undef;
            }
            else
            {
                my $hash = {};
                opendir((my $dh), $path);
                my @new_paths = map  { catfile $path , $_ }
                                grep { !/^\.\.?\z/ } readdir $dh;

                unshift @queue, map { [$_, $hash] } @new_paths;
                $hash;
            }
        };
    }
    continue
    {
        show_queue(++$count, \@queue) if DD;
    }

    $data;
}

sub show_queue
{
    my ($count, $queue) = @_;

    print "($count) \@queue:\n";
    DD == 2 ? dd $queue : print Dumper($queue);
    print '-' x 50, "\n";
}

##</code><code>##

14:34 >perl 1620_SoPW.pl
(0) @queue:
$VAR1 = [
          [
            './test',
            {}
          ]
        ];
--------------------------------------------------
(0) $next:
$VAR1 = [
          './test',
          {}
        ];
==================================================
(1) @queue:
$VAR1 = [
          [
            'test\\dir1',
            {}
          ],
          [
            'test\\file1',
            $VAR1->[0][1]
          ],
          [
            'test\\file2',
            $VAR1->[0][1]
          ]
        ];
--------------------------------------------------
(1) $next:
$VAR1 = [
          'test\\dir1',
          {}
        ];
==================================================
(2) @queue:
$VAR1 = [
          [
            'test\\dir1\\file3',
            {}
          ],
          [
            'test\\file1',
            {
              'dir1' => $VAR1->[0][1]
            }
          ],
          [
            'test\\file2',
            $VAR1->[1][1]
          ]
        ];
--------------------------------------------------
(2) $next:
$VAR1 = [
          'test\\dir1\\file3',
          {}
        ];
==================================================
(3) @queue:
$VAR1 = [
          [
            'test\\file1',
            {
              'dir1' => {
                          'file3' => undef
                        }
            }
          ],
          [
            'test\\file2',
            $VAR1->[0][1]
          ]
        ];
--------------------------------------------------
(3) $next:
$VAR1 = [
          'test\\file1',
          {
            'dir1' => {
                        'file3' => undef
                      }
          }
        ];
==================================================
(4) @queue:
$VAR1 = [
          [
            'test\\file2',
            {
              'dir1' => {
                          'file3' => undef
                        },
              'file1' => undef
            }
          ]
        ];
--------------------------------------------------
(4) $next:
$VAR1 = [
          'test\\file2',
          {
            'dir1' => {
                        'file3' => undef
                      },
            'file1' => undef
          }
        ];
==================================================
(5) @queue:
$VAR1 = [];
--------------------------------------------------
RESULT:
$VAR1 = {
          'test' => {
                      'file1' => undef,
                      'file2' => undef,
                      'dir1' => {
                                  'file3' => undef
                                }
                    }
        };

14:34 >