Fix for RT#1804: Anonymous glob breaks when assigned through

Fix for RT#1804: Anonymous glob breaks when assigned through

The problem here is that globs are scalars and the = operator can only
distinguish between scalar and glob assignments by the flags on the
glob. It only sees the return value of *{}, not the *{} itself. We can
fix this by having the pp_sassign look for a rv2gv (*{}) on its LHS,
to decide what type of assignment to do.