I started trying to answer this question a couple of times. The code is pretty straight-forward, and certainly fixing whatever is wrong should be pretty simple.

But I failed to come up with a solution because I don't know what the problem is. It's not that I don't understand how the code works. It's that I don't understand how it works differently from your needs and expectations, and that is because your needs and expectations haven't been expressed clearly and unambiguously enough.

Could you please follow-up by posting how the output this produces differs from the output you require? Maybe you could show the first 30 lines of correct output, and the first 30 lines of incorrect output. Or just explain better what you want to see.

You are probably expecting consecutive index numbers, but you are printing the results of the  $i * $g multiplication. During the first outer loop iteration,  $i = 1, $g = 1 .. 20, you will get what you expect. During the second,  $i = 2, and multiplying it by the range 1 .. 20 results in 2, 4, 6, 8, ... instead of the 21, 22, 23 ... range you probably want.

In simple assignments like this one, it's good to test your loop on paper, until you are sure the method works.

Also, take a look at the range operator. You can write your loops in a much more readable way:

for my $i (1..6) {
    print "$i\n";
}
[download]

Update:

Take a look at the following example. It's not your exactly your homework assignment, but upon closer inspection you will find that it solves the same problem. Notice the range operator, the modulo operator, and think on why the $grp = 5 if $grp == 0; is needed. Modify the code to suit your needs, and good luck with your next assignments.

#!/usr/bin/perl

use warnings;
use strict;

for my $id (1 .. 30) {
    my $grp = $id % 5;
    $grp = 5 if $grp == 0;
    printf ("ID: %03d\t GRP: %d \n", $id, $grp);
}
[download]

¹) from How (not) to ask a question Include Sample Data (I/O)

What do you expect? What do you get?

I updated to strike thru my first question, but I might need to put it back in. If I understand your required output, I don't see it as something to do using nested for loops. Can you fill in a few more sample values?

      my $j = $i * $g;
[download]

use strict;
use warnings;

my $group = 20;

for ( my $id = 0; $id < 120; $id += $group ) {
    for ( my $grp = 1; $grp <= $group; $grp += 1 ) {

        printf "ID: %d\tGRP: %d\n",
          $id + $grp,
          $grp;
    }
}
[download]