using weak refs and requiring 5.6

One of the optimizations I'm adding to HEAD involves caching component 
objects inside other components. Every time a component foo calls a path 
like <& ../bar &>, we do a bunch of work to canonicalize the path, make 
it absolute, see if it is a subcomponent or method call, etc. But in 
static source mode, you are guaranteed that "../bar" will always map to 
the same component object, so you can create a cache entry

    "../bar" => {bar component object}

inside a private component cache for foo.

However, it is important for the values in this cache to be weak 
references, so that we don't create circular refs or hang onto old 
versions of components.

So my questions are:

* Is Scalar::Util::weaken() stable and performant enough to use more 
extensively in Mason? The docs for Perl 5.6.0 say that weak references 
are still experimental, but I never found a corresponding doc to say 
that they are now stable. Does the stability vary depending on Perl 
version? And I saw this performance patch against 5.8 -
http://aspn.activestate.com/ASPN/Mail/Message/perl5-porters/1506613
which I didn't really understand but worried me.

* I remembered vaguely discussing that we're going to require 5.6 
starting with Mason 1.3, so Scalar::Util::weaken should be available 
everywhere, right? Should we require 5.6.0 or 5.6.1? I know that the 
former is problematic and unstable, but lots of default setups still use 
it. I don't want to leave them out in the cold unless we feel it'll be 
especially unstable in conjunction with Mason and weakrefs.

Jon




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