use strict;
use warnings;
use autodie;

my $processes = 0;
my @ids = 1 .. 40;

for my $id (@ids) {
    if ( $processes >= 8 ) {
        wait;
        $processes--;
    }
    if (fork) {
        $processes++;
    }
    else {
        chdir $id;
        exec $command, $id;
    }
}

wait while $processes--;