...
        $str .= $1 . $/ while ( $ARGV[0] =~ m/(.{3})/g );
...

##</code><code>##

...
use constant LEN => 3;
...
        my $offset = 0;
        my $length = LEN;

        while ( $_ = substr $ARGV[0], $offset, $length ) {
            last if length != LEN;
            $str .= $_ . $/;
            $offset += $length;
        }
...

##</code><code>##

#!usr/bin/perl -w
use strict;
use constant LEN => 3;
use Benchmark qw(:all);

my $str;
my $count = -2;

my $re = timethese(
    $count,
    {
    substring => sub {

        my $offset = 0;
        my $length = LEN;

        while ( $_ = substr $ARGV[0], $offset, $length ) {
            last if length != LEN;
            $str .= $_ . $/;
            $offset += $length;
        }
    },

    regex => sub {

        $str .= $1 . $/ while ( $ARGV[0] =~ m/(.{3})/g );
    }
  }
);

cmpthese($re);

##</code><code>##

Benchmark: running regex, substring for at least 2 CPU seconds...
     regex:  1 wallclock secs ( 2.09 usr +  0.00 sys =  2.09 CPU) @ 14030.14/s (n=29323)
 substring:  3 wallclock secs ( 2.14 usr +  0.00 sys =  2.14 CPU) @ 23225.70/s (n=49703)
             Rate     regex substring
regex     14030/s        --      -40%
substring 23226/s       66%        --

##</code><code>##

...
        $str .= $1 . $/ while ( $ARGV[0] =~ m/(.{3})/g ); ## NOTE the number 3
...

##</code><code>##

...
        $str .= $1 . $/ while ( $ARGV[0] =~ m/(.{LEN})/g ); ## NOTE 3 is now LEN
...

##</code><code>##

Benchmark: running regex, substring for at least 2 CPU seconds...
     regex:  4 wallclock secs ( 2.72 usr +  0.01 sys =  2.73 CPU) @ 740317.95/s (n=2021068)
 substring:  2 wallclock secs ( 2.66 usr +  0.02 sys =  2.68 CPU) @ 17332.84/s (n=46452)
              Rate substring     regex
substring  17333/s        --      -98%
regex     740318/s     4171%        --