use Devel::Peek;
$_ = 'blah';
while(/./g){
    Dump $_;
    foo();
    Dump $_;
    last;
}

sub foo {
    local $_ = 'foo';
    pos($_) = 2;
    Dump $_;
}
__END__
output is:

SV = PVMG(0x8102ff8) at 0x80e72ac
  REFCNT = 1
  FLAGS = (SMG,POK,pPOK)
  IV = 0
  NV = 0
  PV = 0x80f3400 "blah"\0
  CUR = 4
  LEN = 5
  MAGIC = 0x80f3660
    MG_VIRTUAL = &PL_vtbl_mglob
    MG_TYPE = 'g'
    MG_LEN = 1
SV = PVMG(0x8103018) at 0x80e7300
  REFCNT = 2
  FLAGS = (SMG,POK,pPOK)
  IV = 0
  NV = 0
  PV = 0x80f2008 "foo"\0
  CUR = 3
  LEN = 4
  MAGIC = 0x80f3660
    MG_VIRTUAL = &PL_vtbl_mglob
    MG_TYPE = 'g'
    MG_LEN = 2
SV = PVMG(0x8102ff8) at 0x80e72ac
  REFCNT = 1
  FLAGS = (SMG,POK,pPOK)
  IV = 0
  NV = 0
  PV = 0x80f3400 "blah"\0
  CUR = 4
  LEN = 5
  MAGIC = 0x80f3660
    MG_VIRTUAL = &PL_vtbl_mglob
    MG_TYPE = 'g'
    MG_LEN = -1


##</code><code>##

use Devel::Peek;
$_ = 'blah';
while(/./g){
    Dump $_;
    for ('foo'){      # implicit localization of $_
        pos($_) = 2;
        Dump $_;
    }
    Dump $_;
    last;
}
__END__
output:

SV = PVMG(0x8102ff8) at 0x80e72ac
  REFCNT = 1
  FLAGS = (SMG,POK,pPOK)
  IV = 0
  NV = 0
  PV = 0x80f4d78 "blah"\0
  CUR = 4
  LEN = 5
  MAGIC = 0x80f35d0
    MG_VIRTUAL = &PL_vtbl_mglob
    MG_TYPE = 'g'
    MG_LEN = 1
SV = PVMG(0x8103018) at 0x80f90f0
  REFCNT = 3
  FLAGS = (SMG,POK,READONLY,pPOK)
  IV = 0
  NV = 0
  PV = 0x80f38e8 "foo"\0
  CUR = 3
  LEN = 4
  MAGIC = 0x80f1cb0
    MG_VIRTUAL = &PL_vtbl_mglob
    MG_TYPE = 'g'
    MG_LEN = 2
SV = PVMG(0x8102ff8) at 0x80e72ac
  REFCNT = 1
  FLAGS = (SMG,POK,pPOK)
  IV = 0
  NV = 0
  PV = 0x80f4d78 "blah"\0
  CUR = 4
  LEN = 5
  MAGIC = 0x80f35d0
    MG_VIRTUAL = &PL_vtbl_mglob
    MG_TYPE = 'g'
    MG_LEN = 1

##</code><code>##

$main::var = 12;
{
    local $main::var = 10;
    $main::bar = \$main::var;
}
$$main::bar = 42;        # doesn't affect $main::var
print $main::var;        # prints: 12