OK. So I can now replace one colour with another. I am now trying to make the replacement colour transparent. Say I want to replace white with red; I have the following code:

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

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

$img->map(red=>$redmap, blue=>$blumap, green=>$grnmap);
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This replaces all instances of (255,255,255) with (255,0,0). Now how do I make that red transparent? I thought I would need to do:

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);
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But that doesn't do anything. I like Imager for it's flexibility, but I am really getting frustrated with the documentation. Any help would be greatly appreciated.

Cheers!
Aaron