Earlier and later ActiveState builds of Perl 5.8.8 both support weaken:

Build 817:

>perl -e"use Scalar::Util qw( weaken );"

>
[download]

Build 824:

>perl -e"use Scalar::Util qw( weaken );"

>
[download]

So either

• ActiveState messed up for build 820.,
• you have a botched install (Util.dll isn't in lib/auto/List/ or site/lib/auto/List/), or
• something is causing perl to be unable to locate Util.dll in lib/auto/List/or site/lib/auto/List/