$user_string = "[SYN, ACK]";
if ($x =~ /\Q$user_string\E/) {
   print("match\n");
} else {
   print("no match\n");
}

##</code><code>##

$user_string = "[SYN, ACK]";
if ($x =~ /(\[.*?\])/) {
   if ($1 eq $user_string) {
      print("match\n");
   } else {
      print("no match\n");
   }
} else {
   print("not even close\n");
}