Rate substr  regex
substr 481/s     --   -14%
regex  558/s    16%     --

##</code><code>##

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

my @list;
{
    use bigint;
    
    my @chars = ('0' .. '9', 'a' .. 'f');
    
    @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  regex substr
regex   5672/s     --   -80%
substr 28469/s   402%     --