So shouldn't @_ say 0?

Arguments are passed by reference but in your case you end up with an array of one element, that one element being a reference to the non-existent element of @ARGV. So, @_ has one element even though that element is a reference to a non-existent element of @ARGV.

Consider the following example:

use strict;
use warnings;
use Devel::Peek;

Dump(\@ARGV);

mysub($ARGV[0], $ARGV[1]);

mysub(@ARGV);

exit(0);

sub mysub {
    print "mysub\n";
    Dump(\@_);
}
 
__END__

SV = RV(0x83591fc) at 0x83591f0
  REFCNT = 1
  FLAGS = (TEMP,ROK)
  RV = 0x83693c0
  SV = PVAV(0x835a228) at 0x83693c0
    REFCNT = 2
    FLAGS = ()
    ARRAY = 0x0
    FILL = -1
    MAX = -1
    ARYLEN = 0x0
    FLAGS = (REAL)
mysub
SV = RV(0x83590dc) at 0x83590d0
  REFCNT = 1
  FLAGS = (TEMP,ROK)
  RV = 0x8376420
  SV = PVAV(0x835a28c) at 0x8376420
    REFCNT = 3
    FLAGS = ()
    ARRAY = 0x838a418
    FILL = 1
    MAX = 3
    ARYLEN = 0x0
    FLAGS = (REAL)
    Elt No. 0
    SV = PVLV(0x83871d4) at 0x83591f0
      REFCNT = 2
      FLAGS = (GMG,SMG)
      IV = 0
      NV = 0
      PV = 0
      MAGIC = 0x839e158
        MG_VIRTUAL = &PL_vtbl_defelem
        MG_TYPE = PERL_MAGIC_defelem(y)
      TYPE = y
      TARGOFF = 0
      TARGLEN = 1
      TARG = 0x83693c0
      SV = PVAV(0x835a228) at 0x83693c0
        REFCNT = 3
        FLAGS = ()
        ARRAY = 0x0
        FILL = -1
        MAX = -1
        ARYLEN = 0x0
        FLAGS = (REAL)
    Elt No. 1
    SV = PVLV(0x8387200) at 0x8359340
      REFCNT = 2
      FLAGS = (GMG,SMG)
      IV = 0
      NV = 0
      PV = 0
      MAGIC = 0x839eee8
        MG_VIRTUAL = &PL_vtbl_defelem
        MG_TYPE = PERL_MAGIC_defelem(y)
      TYPE = y
      TARGOFF = 1
      TARGLEN = 1
      TARG = 0x83693c0
      SV = PVAV(0x835a228) at 0x83693c0
        REFCNT = 3
        FLAGS = ()
        ARRAY = 0x0
        FILL = -1
        MAX = -1
        ARYLEN = 0x0
        FLAGS = (REAL)
mysub
SV = RV(0x83591fc) at 0x83591f0
  REFCNT = 1
  FLAGS = (TEMP,ROK)
  RV = 0x83696f0
  SV = PVAV(0x835a23c) at 0x83696f0
    REFCNT = 3
    FLAGS = ()
    ARRAY = 0x83a5f30
    FILL = -1
    MAX = 3
    ARYLEN = 0x0
    FLAGS = (REAL)
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Note in the above that the array @ARGV is located at 0x83693c0 ( SV = PVAV(0x835a228) at 0x83693c0 ). It has no elements (ARYLEN = 0x0).

In the first call to mysub() there are two arguments: elements 0 and 1 or @ARGV. But the array @_ is not a simple array of values (see SV = PVLV(0x83871d4) at 0x83591f0 and following). The array has two elements and each of these have MAGIC and a reference to the @ARGV array. The first element of @_ has TARGOFF 0 and TARGLEN 1. These are offest and length into the array SV = PVAV(0x835a228) at 0x83693c0, which is @ARGV, which still has no elements (ARYLEN = 0x0). The second element is similar to the first but TARGOFF is 1, for element 1 of @ARGV instead of element 0.

So, when you access $_[0], through the magic of the @_ array you get the value of $ARGV[0] and when you access $_[1] you get the value of $ARGV[1], but when you evaluate @_ in a scalar context you get 2 (the size of the @_ array) rather than 0 (the size of the @ARGV array). In this example the @_ array really does have two elements. In your script it really did have one element.

The second call to mysub() has @ARGV as its argument list. This array is flattened and, as it has no elements in it, it is flattened into an empty list. Thus in the second call @_ has no elements.

You might wonder, if perl passes arguments by reference, why, in the second call to mysub(), @_ isn't a reference to @ARGV (i.e. accessing @_ is the same as accessing @ARGV). I wasn't party to the decisions about how this works, so I can't tell you why. I can only tell you that in this case @_ is not the same as @ARGV, other than in the sense that they are both empty.