sub player {
  # ... Initialisation code ... 
  {
    lock $counter;
    ++$sig_counter;
    cond_wait( $sig_counter ) until $sig_counter == 3;
  }
  # ... Playing code ...
}

sub recorder {
  # ... Initialisation code ...
  {
    lock $counter;
    ++$sig_counter;
    cond_wait( $sig_counter ) until $sig_counter == 3;
  }
  # ... Recording code ...
}

$sig_counter = 0;
$player_thread   = threads->create( \&player );
$recorder_thread = threads->create( \&recorder );
{ # This block should wait for threads to init
  lock $sig_counter;
  cond_wait( $sig_counter ) until $sig_counter == 2;
}
{ # This block should signal subs to start main code
  lock $sig_counter;
  ++$sig_counter;
}