The comment by tye translates into the syntax for your example:

$a = new Math::Matrix ([1], [2], [3]);
$x = new Math::Matrix ([4], [5], [6]);

$f=new Math::Matrix([1, 2, 3],[4, 5, 6]);
$f=$f->cross_product;
[download]

Kind of a bizarre syntax. Thanks for everyone who's posted here for helping me put the pieces together to sort this out. I don't know how to write a patch, or I would do so...