I just recently started messing around with threads, so I haven't had alot of experience on the topic, but using semaphores seems like a possible solution. Here's an example:

#!/usr/bin/perl
use strict;
use threads;
use Thread::Semaphore;

my $semaphore = new Thread::Semaphore;
$semaphore->down;

sub code {
  my $job = $_[0];
  $semaphore->down;
  print "$job\n";
}

foreach (1..10) {
  threads->new(\&code, $_);
}

sleep 5;
$semaphore->up(10);
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Each thread stores it's argument, then attempts to decrement the resource count, since the initial $semaphore->down earlier in the script leaves the current resource count at 0, any new attempts at decrementing it cause the thread to block. When you want the threads to run, the resource count is set to 10, allowing the 10 blocked threads to unblock and execute. The code is untested.