No need to surround the \s+ with parenthesis. The only reason to use parenthesis is if you want to "capture" that part of the match. Using your example:

  if( $test_string =~ /hello(\s+)world/ig ) {
     $spaces_between = $1; ### $1 comes from matching the
                           ### expression in parenthesis
     $ct_spaces_between = length $spaces_between;
     print "FOUND IT!  The number of spaces between 'hello' and 'world
+' was $ct_spaces_between\n";
  }
[download]

You should be able to use just /hello\s+world/ as your regular expression.

HTH.