AFAIK, Paul has not spoken on the subject of the fence itself. My guess would be that he supports the building of the fence, as it is law, and there's nothing unconstitutional about it.
i merely refer to Hunter's one majour point. i'm certainly not against the fence, but it's not going to solve our international dilemmas.
Q: You voted to support that 700-mile fence along the border with Mexico. Is there a need for a similar fence along the border with Canada?
PAUL: No. The fence was my weakest reason for voting for that, but enforcing the law was important, and border security is important. And we’ve talked about amnesty, which I’m positively opposed to. If you subsidize something, you get more of it. We subsidize illegal immigration, we reward it by easy citizenship, either birthright or amnesty.
Source: 2007 GOP debate at Saint Anselm College Jun 3, 2007