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use Digest::MD5;

# Given a message and key, returns a message authentication code
...
    my $mac   = compute_mac(int($score), $secret);
    "$score/$mac";
}

use CGI;
use CGI::Cookie;

...
$score     = authenticated_score($score);
my $cookie = CGI::Cookie->new(-name => 'score', -value => $score);
print header(-cookie=>$cookie);

my %cookies = CGI::Cookie->fetch;
$score = $cookies{score}->value;

# Eliminate any score that's been tampered with
$score = 0 unless $score eq authenticated_score($score);

my %cookies = CGI::Cookie->fetch;
my $score = $cookies{score}->value;
$score = 0 unless $score eq authenticated_score($score);
...
print header(-cookie=>$cookie);

# (send the rest of your HTML to the player.)