You could try using the index function. Then you won't have to worry about escaping characters (at least in this scenario):

my $concat = "good times and fun (times) and bad (times)";

for my $string ('good times', 'bad times', 'fun (times)') {
   # if the index function returns -1, it is not a substring
   if (index($concat, $string) == -1) {
      $concat .= " and $string";
   }
}

print $concat . "\n";
[download]