($left,$middle,$right) = $sequence =~ m/(\w{$_})?(\w{1})?(\w*)/;

##</code><code>##

use strict;
use warnings;
use Benchmark qw(cmpthese);
my $iterations    = shift @ARGV || 5000;
my $sequence_size = shift @ARGV || 10;

# Simplistic random sequence
my $sequence = join '', map {chr(65 + rand(26))} 1..$sequence_size;

cmpthese($iterations,{
    re_orig => \&re_orig,
    re_set  => \&re_set,
    bsubstr => \&bsubstr,
});
# Test using the original regex
sub re_orig {
    for my $i (0..length($sequence)-1) {
        my ($left,$middle,$right) = $sequence =~ m/(\w{$i})?(\w{1})?(\w*)/;
    }
}
# Test using the character set [A-Z]
sub re_set {
    for my $i (0..length($sequence)-1) {
        my ($left,$middle,$right) = $sequence =~ m/([A-Z]{$i})([A-Z])([A-Z]*)/;
    }
}

# Test using substr
sub bsubstr {
    for my $i (0..length($sequence)-1) {
        my ($left)   = substr($sequence,0,$i);
        my ($middle) = substr($sequence,$i,1);
        my ($right)  = substr($sequence,$i+1);
    }
}