I am not sure how you are interpreting these results, but the wit is running faster. As to why it isn't the 50% expected, I would suggest you run the following instead:

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

cmpthese( -25, {
  non => sub{my $g = 100_000; my $lim = 100_000;
             for ( my $i = 1; $i < $lim; ++$i ) { --$g }},
  wit => sub{my $g = 100_000; my $lim = 100_000;
             for ( my $i = 1; $i < $lim; ++$i ) { --$lim }}
} );
__DATA__
     Rate  non  wit
non 103/s   -- -50%
wit 206/s  99%   --
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