You might be getting sidetracked into issues with @_. I tried running your examples with:

mysub( $x, "a string" );
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and I got the same results as using =>, however they both differ slightly from yours (aside from the addresses):

SV = RV(0x3b230) at 0x3b224
  REFCNT = 1
  FLAGS = (TEMP,ROK)
  RV = 0x18ab37c
  SV = PVAV(0x3c2f0) at 0x18ab37c
    REFCNT = 3
    FLAGS = ()
    ARRAY = 0x18bae14
    FILL = 1
    MAX = 3
    ARYLEN = 0x0
    FLAGS = (REAL)
    Elt No. 0
    SV = IV(0x18ab3a8) at 0x18ab3ac
      REFCNT = 2
      FLAGS = (PADMY,IOK,pIOK)
      IV = 10
    Elt No. 1
    SV = PV(0x381ec) at 0x18c6944
      REFCNT = 2
      FLAGS = (PADTMP,POK,READONLY,pPOK)  # < difference here
      PV = 0x18c0b9c "a string"\0
      CUR = 8
      LEN = 12
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I'm running Windows ActiveState 5.10.0.