use strict;
use warnings;
use Data::Dumper;
use Text::CSV_XS;

my $csv = Text::CSV_XS->new();      # create a new object
while (<DATA>) {
    my $status  = $csv->parse($_);  # parse a CSV string into fields
    my @columns = $csv->fields();         # get the parsed fields
    print Dumper(\@columns);
}

__DATA__
aa,bb,cc
0,2,3
[download]

which prints:

$VAR1 = [
          'aa',
          'bb',
          'cc'
        ];
$VAR1 = [
          '0',
          '2',
          '3'
        ];
[download]

Then, you could use the column positions to format your ASCII data file, by reading in the data line by line and adding commas as follows:

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
[download]

this prints:

12345,678901,234567890
abcde,fghijk,lmnopqrst
[download]

Perhaps you could adapt these simple examples for your application.

One way is to make a list of the start column for each field, in reverse order, then iterate over your data using substr to insert the commas.

Could we see what you have coded so far? I assume you have at least tried to solve this problem yourself seeing how it explicitly states in the rules that we will not do your homework for you.