But it works.

Er, almost :-(
There is a bug in sub build_tree.
To fix it, you have to replace the  if that pops the stack by a while .
The test below shows the difference.

Rudif

#! perl -w

use strict;
use File::Find;
use File::Path;
use Data::Dumper;
$Data::Dumper::Indent = 1;

my $dir = './buildtreetestdir';
mkpath "$dir";
mkpath "$dir/dir1";
mkpath "$dir/dir1/dir11";
mkpath "$dir/dir2";
mkpath "$dir/dir2/dir21";


build_tree(my $tree, $dir);

build_tree_fixed(my $tree_fixed, $dir);

print Data::Dumper->Dump( [ $tree, $tree_fixed ], [ qw / tree tree_fix
+ed / ] );

{ 
  sub build_tree {
    my $node = $_[0] = {};
    my @s;
    find( sub {
      $node = (pop @s)->[1] if @s and $File::Find::dir ne $s[-1][0];
      return $node->{$_} = -s if -f;
      push @s, [ $File::Find::name, $node ];
      $node = $node->{$_} = {};
    }, $_[1]);
    $_[0]{$_[1]} = delete $_[0]{'.'};
  }
}

{ 
  sub build_tree_fixed {
    my $node = $_[0] = {};
    my @s;
    find( sub {
      $node = (pop @s)->[1] while @s and $File::Find::dir ne $s[-1][0]
+;
      return $node->{$_} = -s if -f;
      push @s, [ $File::Find::name, $node ];
      $node = $node->{$_} = {};
    }, $_[1]);
    $_[0]{$_[1]} = delete $_[0]{'.'};
  }
}

__END__

$tree = {
  './buildtreetestdir' => {
    'dir2' => {
      'dir21' => {},
      'dir1' => {
        'dir11' => {}
      }
    }
  }
};
$tree_fixed = {
  './buildtreetestdir' => {
    'dir1' => {
      'dir11' => {}
    },
    'dir2' => {
      'dir21' => {}
    }
  }
};
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