Well I would go for a different solution to all the others here which all rely on you having pre-calculated the dice scores, OK if you are working with d6 but what if you go to d8 or d12? (giving away my AD&D heritage here :)

for (1..13){
  my $chance = chance_of_2dice_score($_, 6);
  print "Chances of rolling a $_ on two dice is: $chance\n";
}

sub chance_of_2dice_score {
  my $score = shift;
  my $dice  = shift;
  my %valid_dice = map {$_,1} (4,6,8,10,12,20);
  die "Invalid dice $dice" if (! $valid_dice{$dice});
  my $count = 0;
  for my $first (1..$dice){
    for my $second (1..$dice){
      ++$count if (($first + $second) == $score);
    }
  }
  return ($count / ($dice * $dice));
}
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I think that this is clearer than the other solutions because I can see the mechanics of it. Now, does any-one have an algorithm for x dice? :-)

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