my @work = fetch_jobs($starting_condition);
my %seen;
while ( @work ) {
    # Remove 1 item from our stack/queue
    my $job = shift @work;

    # Skip this job if we have already done it
    next if $seen{$job}++;

    # Possibly add new jobs to our stack/queue
    if ( more_jobs($job) ) {
        push @work, fetch_jobs($job);
    }

    # process job
}