[perl #114388] LTA error when using s[] = "rea"

[Christian Bartolomaeus via RT]

The first evaluation fails again (no method 'subst-mutate' nowadays), so I'm 
re-opening this ticket.

$ perl6 -e 's[ea] = "rea";'
Method 'subst-mutate' not found for invocant of class 'Any'
  in block <unit> at -e:1

We're back to "We could improve the error, but what should it say" (moritz, 
2012) and "Maybe it should mention $_ somewhere, since it tries to call subst 
on $_" (FROGGS, 2013).