use strict;
use warnings;

my @cols = qw (5 11);
while (<DATA>) {
    chomp;
    my $row = '';
    my $start = 0;
    for my $cs (@cols) {
        my $offset = $start;
        my $length = $cs-$start;
        $row .= substr($_, $offset, $length) . ',';
        $start = $cs;
    }
    $row .= substr($_, $start);
    print "$row\n";
}

__DATA__
12345678901234567890
abcdefghijklmnopqrst