my $redmap = [0 .. 255];
my $blumap = [0 .. 255];
my $grnmap = [0 .. 255];
my $alpha  = [0 .. 255];

$blumap->[255] = 0;
$grnmap->[255] = 0;
$alpha->[0] = 255;

$img->map(red=>$redmap, blue=>$blumap, green=>$grnmap, alpha=>$alpha);