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

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  1. local $_ is special, though.

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

In reply to Re^6: foreach $1 by ikegami
in thread foreach $1 by choroba

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