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

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

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

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

use strict;
use warnings;

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