I am not sure whether this answers your question or whether the benchmark is correct, but I hope it can help you:

#!/usr/bin/perl
use warnings;
use strict;
use feature 'say';

use bigint;
use Benchmark qw/cmpthese/;

my @chars = ('0' .. '9', 'a' .. 'f');

my @list = map {join q(), map $chars[rand @chars], 1 .. 32} 1 .. 100;
say for @list;
$_ = hex $_ for @list;
say for @list;

cmpthese(0, {
              substr => sub {
                  my @l = @list;
                  substr $_, -2, 0, '.' for @l;
              },
              regex => sub {
                  my @l = @list;
                  s/(..)$/.$1/ for @l;
              }
             });

__END__

        Rate substr  regex
substr 327/s     --   -14%
regex  380/s    16%     --
[download]

Update: See the replies for more correct benchmarks and better solutions. Thanks, kennethk and Eliya.

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