I *think* this quick program does what you need:

use strict;
use warnings;
use Data::Dumper;

my %dependencies;
while (<DATA>) {
    chomp;
    my ($key, @items) = split /,/;
    $dependencies{$key} = [@items];
}
# print Dumper \%dependencies;

my $continue = 1;
while ($continue) {
    $continue = 0;
    for my $key (keys %dependencies) {
        my @jobs = @{$dependencies{$key}};
        my %seen = map { $_ => 1 } @jobs;
        my @add;
        for my $job (@jobs) {
            next unless exists $dependencies{$job};
            my @new_jobs = @{$dependencies{$job}};
            for my $new_job (@new_jobs) {
                next if exists $seen{$new_job};
                $continue = 1;
                push @add, $new_job;
                $seen{$new_job} = 1;
            }
        }
        push @{$dependencies{$key}}, @add;
    }
}
            
print Dumper \%dependencies;
        


__DATA__
job1,
job2,job1,
job3,job2
job4,job2
job5,job2,job4
job6,jobz
joba,job4,jobb
jobz,jobbc,job2
[download]

$VAR1 = {
          'job3' => [
                      'job2',
                      'job1'
                    ],
          'jobz' => [
                      'jobbc',
                      'job2',
                      'job1'
                    ],
          'job6' => [
                      'jobz',
                      'jobbc',
                      'job2',
                      'job1'
                    ],
          'job4' => [
                      'job2',
                      'job1'
                    ],
          'joba' => [
                      'job4',
                      'jobb',
                      'job2',
                      'job1'
                    ],
          'job5' => [
                      'job2',
                      'job4',
                      'job1'
                    ],
          'job1' => [],
          'job2' => [
                      'job1'
                    ]
        };
[download]