[perl #119501] \(1+2) always referencing the same sv

[perl #119501] \(1+2) always referencing the same sv

2484f8dbbb hid the fact that constant folding happens by making 1+2
fold to  a PADTMP, an SV that is never used as an lvalue and gets cop-
ied if one tries to use it that way.

The PADTMP mechanism is what allows \"$x" to return a new value each
time, even though ops like "$x" actually reuse the same scalar repeat-
edly to return values.

Because \ copies PADTMPs, \(1+2) ends up folding 1+2 to 3 (marked
PADTMP), which is then copied by the \ at compile time.  (Constant
folding works by evaluating certain ops at compile time and then
inlining their returned value.)  The result is that we have a folded
\3 where 3 is *not* marked PADTMP (the reference is, instead); hence
\(1+2) gives the same scalar each time, producing a value that can be
modified, affecting future evaluations.

The solution is to skip folding \ if its argument is a PADTMP.