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my %colour_min, %colour_max;
...
  $slider{$channel} = ( $triplet{$channel} - $colour_min{$channel} ) /
+ ( $colour_max{$channel} / $colour_min{$channel} ) );
}

for my $channel ('red', 'green', 'blue')
{
$slider{$channel} = $triplet{$channel} / $colour_max{$channel};
}