my @letters = split //, $pass; # get the letters into 
                               # an array
my %lc = (); # initialize a hash for use in a count.
foreach my $letter (@letters) {
    $lc{$letter}++; # count the occurrence of each letter
}
my @keys = keys %lc; get the individual characters used.
if (($#keys+1)/length($foo) < .6) {
  # if there are less than 60% unique characters
  print "not enough different letters\n";
} 
my $ok = 1; # just a flag
foreach my $key (@keys) {
  # check the frequency of individual letters.
  # if a character occurs more than 20% of the time, it is
  # no good.
  # an alternate way to do this would be to check the actual
  # occurrences of each character:
  # $ok = 0 if ($lc{$key} >= 2);

  $ok = 0 if ($lc{$key}/length($foo) > .2) 

}
print "You have to have more different letters." unless $ok;