Your example is misleading since the two snippets actually do different things. Check $; in perldoc perlvar. It is used to be used to create "multidimensional" hashes, but now we have better means, i.e. references, to do so.

In general you should not be too worried about micro-optimizations. Indeed people is often advised against doing so.

Oh! and the instructive bit in this example is that B::Deparse is precisely helping you to notice the difference between the two constructs, not to discover "compile time optimizations"...