HI,

If the length of the string is greater than 20 then that string is ignored and skipped to next string.

while (<DATA>) {
   unless (length($_) > 20 ) {
     if ($_=~m#^[a-zA-Z]+(\s?[a-zA-Z]+)?$#) {
        print $_ . "\n";
     }
  }
}
__DATA__
Johnson
Larry Wall
GeorgeWashingtonBushBush
[download]

Results:

Johnson
Larry Wall