$diff = abs($r1 - $r2) + abs($g1 - $g2) + abs($b1 - $b2);

##</code><code>##

$diff = ($r1 - $r2) ** 2 + ($g1 - $g2) ** 2 + ($b1 - $b2) ** 2;
(or I can make this more std like)

##</code><code>##

my $ll = 0.212671 * $r + 0.715160 * $g + 0.072169 * $b;