Recursion and a "been there" flag is a clean way to handle the problem:

use strict;
use warnings;
use Data::Dump;

my %depends;

while (<DATA>) {
    chomp;
    
    my ($job, @deps) = split ',';
    
    $depends{$job}{$_} = undef for @deps;
}

addDepends (\%depends, $_, keys %{$depends{$_}}) for keys %depends;

print Data::Dump::dump(\%depends);


sub addDepends {
    my ($depends, $target, @deps) = @_;
    
    for my $dep (@deps) {
        next if $depends->{$target}{$dep}++;
        
        addDepends($depends, $target, keys %{$depends{$dep}});
    }
}

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

Prints:

{
  job1  => {},
  job2  => { job1 => 1 },
  job3  => { job1 => 1, job2 => 1 },
  job4  => { job1 => 1, job2 => 1 },
  job5  => { job1 => 2, job2 => 2, job4 => 1 },
  job6  => { job1 => 2, job2 => 1, jobbc => 1, jobz => 1 },
  joba  => { job1 => 1, job2 => 1, job4 => 1, jobb => 1 },
  jobb  => {},
  jobbc => {},
  jobz  => { job1 => 1, job2 => 1, jobbc => 1 },
}
[download]