Oh ok, right. I didn't intend to be that general. There are a few select places where execution order is defined. Your example of the RHS computing before the LHS isn't a good one because that part isn't well defined. That only matters where computing the LHS has some sort of external effect on a dependancy of the RHS. Of course, the RHS has to be executed before the assignment but the LHS also has to execute to find the lvalue for the assignment and its that part that can move around.

Anyhow, I think I'm clear on when I care about execution order - only when the sub-expressions in a statement depend on each other - say multiple shift statements: sub { ${ shift() } = shift }->( a, b ). In this case sassign wants the LHS as its last argument so the second shift runs first then the first shift and scalar dereference. Execution order matters here because a common element is being altered. Ah well.

perl -MO=Concise -e '@ARGV=qw(a b); ${shift()} = shift;' | Idealized optrees from B::Concise

leave
   enter
   nextstate
   aassign
      pushmark
      const   a
      const   b
      pushmark
      rv2av
         gv
   nextstate
   sassign
      shift
         rv2av
            gv
      rv2sv
         scope
            shift
               rv2av
                  gv
[download]

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