sub calculate_score {
    my $word         = shift();
    my $score        = 0;
    my %letter_value = ();
    my @letters      = split( '', $word );
    foreach ( @letters ) {
        if ( defined( $letter_value{$_} ) ) {
           $letter_value{$_} = $letter_value{$_} * 2;
           $score += $letter_value{$_};
         } else {
           $score += 1;
           $letter_value{$_} = 1;
        }
    }
    return $score;
}