Hello asqwerty, and welcome to the Monastery!

You already have the answer you were looking for. I just want to point out that the line:

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

is incorrect. You can see this by printing the contents of %dfs (using Data::Dumper) immediately after that line:

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

When interpolated into an array or hash, the empty list () effectively disappears. I think you meant to write:

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

which works correctly, although

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

also works.

Here is some good advice: Get into the habit of including the lines

use strict;
use warnings;
[download]

at the head of every script. In this case, warnings would have shown you the problem:

Odd number of elements in hash assignment at ... line 6.
[download]

Hope that helps,