while (
   my ($title, $what, $date, $source, $data) = $string =~ m{
      (\w+)\n                     # Grab the title
      What:\s+([^\n]+)\n          # Grab the what
      Date\sadded:\s+([^\(^\n]+)  # Date added
      (\([^\n]+\))?\n             # Optional source
      Data:\s+([^\n]+)\n          # Grab the data
   }xmgs
) {
   $source = '' if not defined $source;  # Use default ''.

   ...
}