has 'balance' => ( isa => 'Int', is => 'rw', default => 0 );
This says that a B<BankAccount> has a C<balance> attribute, which has
-a C<Int> type constraint, a read/write accessor, and a default value
+an C<Int> type constraint, a read/write accessor, and a default value
of C<0>. This means that every instance of B<BankAccount> that is
created will have its C<balance> slot initialized to C<0>, unless some
other value is provided to the constructor.