my %dispatch = (
  From    => sub { return '"Nobody" <nobody@localhost>'; },
  Subject => sub { die "The GNKSA requires that the subject field always be specified by the user."; },
  # and so forth.
);

##</code><code>##

   for my $k (keys %dispatch) {
      if (exists $config{$k}) {
         $value{$k} = $config{$k};
      } else {
         $value{$k} = $dispatch{$k}->();
      }
   }