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