I'd probably go the unpack route, but walking down substrings is sometimes faster.

use constant CHUNK => 8;

for (my $x = 0; $x < length($str) / CHUNK; ++$x) {
    print substr($str, $x * CHUNK, CHUNK), "\n";
}
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For people who have an aversion to C-style loops in Perl, the above can also be recast as a statement modifier:

print substr($str, $_ * CHUNK, CHUNK), "\n"
    for 0 .. length($str) / CHUNK;
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