IMHO my post should have helped you as it showed that you can extend the expression in the grep codeblock.

But now we know a little more of your problem and I assume the following: If $css->{xlink} is defined and not empty you want to try a match. The otherwise clause is not obvious to me, but you can change it. E.g.

grep { $css->{xlink} && ! /$css->{xlink}/ }
[download]

I hope I understood it right.

McA