$![1]</code< (this is where we store the number of moves) and 3 is zero (i.e. every three moves), call <i>x</i>. <i>x</i> is one of the autogenerated subs that does some substitution on $_. It substitutes the first value passed to it with the second, until there are no values left.<br>
What gets passed to <i>x</i> is <code>$![6]