Borrowing heavily from Kenosis' code (thanks), regex seems to be faster than unpack (at least using substitution):

             Rate _substr _unpack  _regex  _split
_substr 2187335/s      --    -11%    -16%    -20%
_unpack 2457294/s     12%      --     -6%    -10%
_regex  2612321/s     19%      6%      --     -4%
_split  2726283/s     25%     11%      4%      --
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Perl code:

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

my $string = join '', 'A' .. 'Y';

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

sub _regex {
    my @arr;
    while (length $string){ $string =~ s/^(.{5})//; push @arr, $1; }
}

sub _split {
    my @arr = split /.{5}\K/, $string;
}

sub _substr {
    my @arr;

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

cmpthese(
    -5,
    {
        _unpack => sub { _unpack() },
        _split  => sub { _split() },
        _substr => sub { _substr() },
        _regex  => sub { _regex() }
    }
);
[download]