sub split_len {
    my ($str, $start, $len) = @_;
    my @ret;

    for (my $strlen = length $str; $start <= $strlen; $start += $len) {
        push @ret, substr $str, $start, $len;
    }
    return @ret;
}

my $c =  join '', 'a'..'z';
print "@{[ split_len $c, 0, 3 ]}\n";
print "@{[ split_len $c, 0, 4 ]}\n";
print "@{[ split_len $c, 3, 4 ]}\n";
__END__
abc def ghi jkl mno pqr stu vwx yz
abcd efgh ijkl mnop qrst uvwx yz
defg hijk lmno pqrs tuvw xyz