-- Dan

This, and many other questions about color, are answered (though sometimes not directly) in Poynton's Color FAQ. Look for "24. Is CMY just one-minus-RGB?".

# see http://adaptiveview.com/cw/doc5a.html
# red   = 255 - minimum(255,((cyan/255)    * (255 - black) + black))
# green = 255 - minimum(255,((magenta/255) * (255 - black) + black))
# blue  = 255 - minimum(255,((yellow/255)  * (255 - black) + black))

use strict;
use warnings;

# some test data
my @cmyk = qw(0 128 255 127 );

# convert
my @rgb = cmyk2rgb(@cmyk);

# result
foreach (0..2){
  print "$_ $rgb[$_]\n";
}

sub cmyk2rgb {
  my (@cmyk) = @_;
  my $bk = $cmyk[3];
  my $wh = 255 - $bk;
  my @rgb=();
  my $tmp=0;
  for (0..2){
    $tmp = ( ($cmyk[$_]/255) * $wh ) + $bk;
    $tmp = ($tmp > 255) ? 255 : $tmp; 
    $rgb[$_] = 255 - int ($tmp + 0.5);
  }
  return @rgb;
}
[download]

Using map allows for code that is (IMHO) somewhat clearer and more concise.

sub cmyk2rgb {
    my ($cyan, $magenta, $yellow, $black) = @_;
    my $white = 255 - $black;

    use integer;
    return map $_ > 255 ? 0 : 255 - $_
        => map $_ * $white / 255 + $black
        => ($cyan, $magenta, $yellow);
}
[download]

— Arien

This may be of some use to you.