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sub rgb2n{ unpack 'N', pack 'CCCC', 0, @_ }

...
        $v < $_ and return rgb2n( $map{ $_ }->( $v % 255 / 256 ) ) for
+ @map;
    }
}

#! perl -slw
use strict;
...
magenta -> white        255     0..255  255         256