The following is the post I prepared yesterday evening (after 2 a.m. this morning in fact), but I guess I was too tired: I previewed it, but stupidly forgot to hit the create button.

OK, now i ran a benchmark with the various possibilities. The results might be useful to others.

I tried 8 different solutions: a "C_style" solution (converting the string into an array of individual characters), two regexes (one with /\w{5}/ and one with /.{5}/, one with the opening of a file handler on a reference to the string, two variations on a loop with the substr function, the split solution offered by Kenosis (although I probably would not be able to use it with the old version of Perl that we have on our servers, but I could test it at home on my more recent version) and unpack.

The following is the code (borrowed in part from Kenosis):

use strict;
use warnings;
use Benchmark qw/cmpthese/;

my $string = (join '', 'a'..'z') x 10;

my $unpack = sub {
    my @sub_fields = unpack '(A5)*', $string;
};

my $regex1 = sub {
    my @sub_fields = $string =~ /\w{5}/g;
};

my $regex2 = sub {
    my @sub_fields = $string =~ /.{5}/g;
};

my $split = sub { # suggested by Kenosis
    my @sub_fields = split /.{5}\K/, $string;
};

my $substr1 = sub {
    my @sub_fields;
    for ( my $i = 0 ; $i < length $string ; $i += 5 ) {
        push @sub_fields, substr $string, $i, 5;
    }
};

my $substr2 = sub {
    my @sub_fields;
    my $max = (length $string)/5 -1;
    push @sub_fields, substr $string, $_*5, 5  for (0..$max);
};

my $filehandle = sub {
    my (@sub_fields, $var);
    open my $FH, "<", \$string or die "cannot open $string $!";
    push @sub_fields, $var while read $FH, $var, 5;
};

my $c_style_string = sub { # the idea suggested by boftx
    my @sub_fields;
    my @chars = split //, $string;
    while (@chars) { push @sub_fields, join '', splice (@chars, 0, 5)}
+;
};

cmpthese( -1,
    {     regex1  => sub {$regex1->()},
        regex2  => sub {$regex2->()},
        unpack => sub {$unpack->()},       
        split  => sub {$split->()},
        substr1 => sub { $substr1->()},
        substr2 => sub {$substr2->()},
        FH => sub {$filehandle->()},
        C_Style => sub { $c_style_string->()}
    }
)
[download]

           Rate C_Style  regex1  regex2      FH   split substr1 substr
+2  unpack
C_Style  5598/s      --    -72%    -72%    -78%    -79%    -80%    -80
+%    -83%
regex1  19690/s    252%      --     -1%    -24%    -28%    -28%    -31
+%    -39%
regex2  19968/s    257%      1%      --    -23%    -26%    -27%    -30
+%    -38%
FH      25984/s    364%     32%     30%      --     -4%     -5%     -9
+%    -20%
split   27161/s    385%     38%     36%      5%      --     -1%     -5
+%    -16%
substr1 27355/s    389%     39%     37%      5%      1%      --     -4
+%    -16%
substr2 28523/s    409%     45%     43%     10%      5%      4%      -
+-    -12%
unpack  32428/s    479%     65%     62%     25%     19%     19%     14
+%      --
[download]

Update this evening (Jan 31, 2014 at 18:45): I incorporated the unpack solution in my program at work today, and the speed gain I obtained on my real data is significantly better than what could be derived from the figures of the benchmark above. The profiling shows that the modified code line runs surprisingly almost twice faster than the original one.