%dfs = map {$_ => ()} @pref;

##</code><code>##

$VAR1 = {
          'brain' => 'lung',
          'kidney' => undef
        };

##</code><code>##

%dfs = map {$_ => []} @pref;

##</code><code>##

%dfs = map {$_ => undef} @pref;

##</code><code>##

use strict;
use warnings;

##</code><code>##

Odd number of elements in hash assignment at ... line 6.