use Tie::NumRange;
my ($r,$g,$b);
tie $r, Tie::NumRange => (0, 0, 255);
tie $g, Tie::NumRange => (0, 0, 255);
tie $b, Tie::NumRange => (0, 0, 255);

$r = 100;  # fine
$g = 200;  # fine
$b = 300;  # set to 255

$b -= 150;  # fine, is now 105
$g -= 150;  # fine, is now 50
$r -= 150;  # set to 0