Using the formulae from the page zigdon suggested ;

# 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;
}
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poj