Not a bad point, but I'd stick to having a brightness value passed, not a scaling factor. It's a bit easier to have a rough idea of the outcome that way IMO.

sub add_to_hexrgb {
    my @rgb = map hex, unpack "A2"x3, $_[0];
    my $max = (sort {$a <=> $b} @rgb)[-1];
    my $smax = $max + $_[1];

    return
        $smax < 0 ? "000000" :
        $smax > 255 ? "FFFFFF" :
        do {
            my $fact = ($max + $sumd) / $max;
            sprintf "%02X"x3, map { int($_ * $fact) } @rgb
        };
}
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Makeshifts last the longest.