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

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

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

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