C:\>type test.pl
use Parallel::ForkManager;
$|++;   # unbuffer output

my @collections = qw (col1 col2 col3 col4 col5);
my $max_tasks = 3;
$pm = new Parallel::ForkManager($max_tasks);

for my $collection (@collections) {
    my $pid = $pm->start and next; 

    # do something with $collection in kid
    do{print "$_\t$collection\n"; sleep 1 } for 1..2;

    # kill kid
    $pm->finish;
}

C:\>perl test.pl > out

C:\>type out
1       col1
1       col2
1       col3
2       col1
2       col2
2       col3
1       col4
1       col5
2       col4
2       col5

C:\>