Aside from the fact that Perl itself uses them, $_, $a, $b and $" aren't special.^[1] They are just ordinary package variables.

$/ and $\ are special: They are magical vars. I don't know why they are magical vars.

$1 is special: It's a magical var. It's also implicitly localized.

$ perl -MDevel::Peek -e'
   for my $sym (qw( _ a " / \ 1 )) {
      warn "\$$sym\n";
      Dump($$sym);
      warn "\n";
   }
'
$_
SV = NULL(0x0) at 0x7fffc959c850
  REFCNT = 1
  FLAGS = ()

$a
SV = NULL(0x0) at 0x7fffc95545e0
  REFCNT = 1
  FLAGS = ()

$"
SV = PV(0x7fffc9554d40) at 0x7fffc95796f0
  REFCNT = 1
  FLAGS = (POK,pPOK)
  PV = 0x7fffc957e030 " "\0
  CUR = 1
  LEN = 10

$/
SV = PVMG(0x7fffc95732b0) at 0x7fffc95544f0
  REFCNT = 1
  FLAGS = (GMG,SMG,POK,pPOK)
  IV = 0
  NV = 0
  PV = 0x7fffc9574ba0 "\n"\0
  CUR = 1
  LEN = 10
  MAGIC = 0x7fffc9574b40
    MG_VIRTUAL = &PL_vtbl_sv
    MG_TYPE = PERL_MAGIC_sv(\0)
    MG_OBJ = 0x7fffc95544d8
    MG_LEN = 1
    MG_PTR = 0x7fffc9574b80 "/"

$\
SV = PVMG(0x7fffc95b9a80) at 0x7fffc95797f8
  REFCNT = 1
  FLAGS = (GMG,SMG)
  IV = 0
  NV = 0
  PV = 0
  MAGIC = 0x7fffc957aba0
    MG_VIRTUAL = &PL_vtbl_sv
    MG_TYPE = PERL_MAGIC_sv(\0)
    MG_OBJ = 0x7fffc95797e0
    MG_LEN = 1
    MG_PTR = 0x7fffc958fb70 "\\"

$1
SV = PVMG(0x7fffc9573a00) at 0x7fffc9584420
  REFCNT = 1
  FLAGS = (GMG,SMG)
  IV = 0
  NV = 0
  PV = 0
  MAGIC = 0x7fffc9591e50
    MG_VIRTUAL = &PL_vtbl_sv
    MG_TYPE = PERL_MAGIC_sv(\0)
    MG_OBJ = 0x7fffc9584408
    MG_LEN = 1
[download]

1. local $_ is special, though.

Update: Added mention that local $_ is special and that $1 is implicitly localized. Added code.

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
[download]

> $_, $a, $b and $" aren't special.

Where do you get this definition of "special" from, they are listed under perlvar#SPECIAL-VARIABLES.

The docs specify the terminology

> $/ and $\ are special: They are magical vars. ... $1 is special: It's a magical var

OK, so your point seems to be that "magical vars", i.e. variables with a MAGIC attribute fail to work as expected as loop variables.

So you agree that at least those vars should raise a warning in this case?

Cheers Rolf
_{(addicted to the Perl Programming Language :)
Wikisyntax for the Monastery}

Where do you get this definition of "special" from

They are special in the sense that they are used by some Perl builtins, but the scalar itself is just an ordinary scalar. You can check using Devel::Peek's Dump. (As shown.)

So you agree that at least those vars should raise a warning in this case?

No. Warning when a magical var is used as the loop iterator would issue far too many incorrect warnings. Most of the time, the fact that the aliased variable doesn't have the original magic is a good thing.

sub g {
   for (qw( abc def )) {
      # $ENV{PATH} didn't get changed.
   }
}

sub f {
   for (@_) {
      # $_ now aliased to $ENV{PATH}
      g();
   }
}

f($ENV{PATH});   # A magical var
[download]