I'm currently reading the book Higher Order Perl and I'm on the section where it covers memoization. The book says a single multiplication won't be improved on by memoization because of the overhead involved and because multiplication is already fast. I'm surprised that your lookup with ord() and substr() is faster than a single multiplication. The only difference I see is that my function is using a float constant and the ord() and substr() don't need to do float calculations.

#!/usr/bin/env perl
use warnings;
use strict;
use Benchmark 'cmpthese';

my $lookup = join '', map{ chr( $_ / 255 * 100 ) } 0 .. 255;
my $const = 100 / 255;

cmpthese(-2, {
    lookup => sub {
        my @output = map { ord( substr $lookup, $_, 1 )} 0 .. 255;
    },
    calc   => sub {
        my @output = map {$_ / 255 * 100} 0 .. 255;
    },
    calc2   => sub {
        my @output = map {$_ * $const} 0 .. 255;
    },
});

my @output1 = map { ord( substr $lookup, $_, 1 )} 0 .. 5;
my @output2 = map {$_ * $const} 0 .. 5;
print "@output1\n";
print "@output2\n";
__END__

# Results on my machine (v5.22.1 built for MSWin32-x64-multi-thread):
          Rate   calc  calc2 lookup
calc   12009/s     --   -24%   -31%
calc2  15753/s    31%     --    -9%
lookup 17376/s    45%    10%     --
0 0 0 1 1 1
0 0.392156862745098 0.784313725490196 1.17647058823529 1.5686274509803
+9 1.96078431372549
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