'ab' =~ /a(z?)b/   # Returns ''.
'ab' =~ /a(z)?b/   # Returns undef.

##</code><code>##

my @fizzbin = ( $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);

##</code><code>##

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 ''.

   ...
}