my $t='the quick red fox jumped over the lazy brown dog';
print count_vowels($t);

sub count_vowels {
   my $string = shift;

   # Check degenerate case: a single character string.
   if (length($string) == 1) {
      return 1 if $string =~ /a|e|i|o|u/i;
      return 0;
   }

   # Break the problem into two smaller problems:
   my ($part1, $part2) = (substr($string,0,1), substr($string,1) );
   return count_vowels($part1) + count_vowels($part2);
}