Do it manually. Autobundle works great when it works, which for some folks is rarely (same goes for CPAN).
For many modules you have to go throught lots of hoops to compile, so Autobundle won't help you much, so like I said earlier, do it manually (check out Compiling Tk::PNG on win32 (complications, more c/xs than perl) for an idea about the tons of pains one can go through).
And last, but not least, why would you want to "convert" a 5.6.1 installation to an equivalent 5.8.0? Just add 5.8.0, so you can have both (at least till you bring your 5.8.0 install up to par).
Do it manually, it feels better.
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** The Third rule of perl club is a statement of fact: pod is sexy. | 