#! perl -slw
use strict;
use threads;
use threads::shared;

my $running :shared = 0;

sub thread {
    { lock $running; ++$running }
    my $subdir = shift;
    chdir $subdir;
    system q[some command ];
    { lock $running; --$running }
}

my @subdirs = ...;

for my $subdir ( @subdirs ) {
    async( \&thread, $subdir );
    sleep 1 while $running >= 8;
    $_->join for threads->list(threads::joinable);
}