It would be better to match what you will accept rather than what you will not accept. It's easier that way and will make your code easier to read and understand.

if ($input =~ m/^[A-Za-z]*\s{0,1}[A-Za-z]*$/) {
  print "match!\n";
} else {
  print "no match!\n";
}
[download]

Now this may not be exactly what are you testing for (i.e. you might want to exclude leading or trailing spaces), but this gives you something to work with. I'll leave the rest up to you and you need help with that, just holler...