You are better off creating a new hash, since both keys and values are being modified.

my %newhash = map { my $list=$_;
                     "\[mailto:$list\@example.com $list]" => 
                      { map { $_ => $_ eq 'desc' ? check_desc $lists{$
+list}{desc}
                                                 : $lists{$list}{$_}
                      } keys %{$lists{$list}
                  } keys %lists;
[download]

Notes:
1 ) Untested
2) This is a Shallow copy of the inner hash.
3) '$list' is a confusing name for the key