use strict;
use warnings;
use List::Util 'sum';

sub measure {
    my @series = @_;
    my %count;
    $count{$_}++ for @series;
    my $norm = sum values %count;
    $_ /= $norm for values %count;
    my %score = %count;

    my $metric = 0;
    for my $elem (@series) {
        $score{$elem} -= 1;
        for (keys %score) {
            $metric += $score{$_}**2;
            $score{$_} += $count{$_};
        }
    }
    
    return $metric/@series;
}

print measure("A", "C", "B", "A", "D", "C", "A", "B", "C", "A"), "\n";
print measure("A", "C", "B", "A", "C", "D", "A", "B", "C", "A"), "\n";
print measure("A", "A", "A", "A", "B", "B", "C", "C", "C", "D"), "\n";