Higher-Order Functions

        do_with_board(sub {
          if ($move eq "$_[0]$_[1]") {
             $squares->[$_[0]][$_[1]] = "x";   ## define array element for choice
          }
        });

This is a little cluttered
If we define do_with_board like this:

        sub do_with_board(&) { ... }

We get a syntax that is like the grep and map operators:

        do_with_board {
          if ($move eq "$_[0]$_[1]") {
             $squares->[$_[0]][$_[1]] = "x";   ## define array element for choice
          }
        };