It's likely %ENV is tied (I am guessing) or has mechanism similar to tie associated with it to pass lookups to an underlying system call

That mechanism is called "magic". Tied variables are a specific kind (MG_TYPE) of magical variables.

%ENV appears to be magical only under shells where the lookup and expansion of environmental variables is a case-insensitve operation.

It is my understanding that %ENV is magical on all platforms. The magic translates access to %ENV into C library calls.

To see if %ENV is magical, run

perl -MDevel::Peek -e 'Dump \%ENV, 1'
[download]

On Windows, that outputs

SV = RV(0x183a4ec) at 0x2251d8
  REFCNT = 1
  FLAGS = (TEMP,ROK)
  RV = 0x225db4
  SV = PVHV(0x229584) at 0x225db4
    REFCNT = 2
    FLAGS = (SMG,RMG,SHAREKEYS)
    IV = 29
    NV = 0
    MAGIC = 0x182a964                            <----
      MG_VIRTUAL = &PL_vtbl_env
      MG_TYPE = PERL_MAGIC_env(E)
    ARRAY = 0x18257cc  (0:13, 1:11, 2:6, 3:2)
    hash quality = 102.6%
    KEYS = 29
    FILL = 19
    MAX = 31
    RITER = -1
    EITER = 0x0
[download]

I don't have access to a unix machine at the moment.

Perl probably doesn't even care whether the env vars are case-sensitive or not. That's probably handled by the C library or passed on the OS.