Well unless theres a version issue going on here then I would write that benchamrk like this

use Benchmark 'cmpthese';
for $size (2, 200, 20_000) {
    $s = "." x $len;
    print "\nDATASIZE = $size\n";
    cmpthese -5,{
        chop2   => '$t = $s; chop $t; chop $t;',
        subs    => '($t = $s) =~ s/..\Z//s;',
        lsubstr => '$t = $s; substr($t, -2) = "";',
        rsubstr => '$t = substr($s, 0, length($s)-2);',
    };
}
[download]

There are a few more ways, but this is a direct but less verbose copy of what you posted. The -5 argument indicates that the benchmarking for each item should take at minimum 5 seconds. (It may take longer) If you used a positive argument then it does that many runs of the given code. It will warn if you dont use enough iterations for it to get a "reasonable" sample.

Please consult the Benchmark documentation as I suspect the interface has moved on since the edition of Programming Perl that you are using. I say this because your code does work, but it looks like Benchmark has been updated to do that idiom automatically.

cheers

--- demerphq
my friends call me, usually because I'm late....