Oh, and why does that matter? Well, since $" isn't special, Perl actually uses $" when you interpolate an array in double-quotes. It doesn't matter that the loop made $" refer to a different SV. But it does matter for magical vars. Since the SV that replaces $\ doesn't have the magic that $\, the underlying variable that Perl actually uses (instead of $\) isn't updated when $\ is changed.

$ perl -MDevel::Peek -e'Dump($\); for $\ (1) { Dump($\); }'
SV = PVMG(0x7fffc3c4baa0) at 0x7fffc3c15ab0
  REFCNT = 1
  FLAGS = (GMG,SMG)
  IV = 0
  NV = 0
  PV = 0
  MAGIC = 0x7fffc3c3b9e0
    MG_VIRTUAL = &PL_vtbl_sv
    MG_TYPE = PERL_MAGIC_sv(\0)
    MG_OBJ = 0x7fffc3c15a20
    MG_LEN = 1
    MG_PTR = 0x7fffc3c3c4f0 "\\"
SV = IV(0x7fffc3c15a40) at 0x7fffc3c15a50
  REFCNT = 2
  FLAGS = (IOK,READONLY,PROTECT,pIOK)
  IV = 1
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