Elements of @_ are aliased to EXISTING/DEFINED elements of the passed parameter.

perl -MData::Dump::Streamer -le '@ary = (); x($ary[5],"foo"); sub x { 
+$_[0] = $_[1] }; Dump(\@ary)'
$ARRAY1 = [
            ( undef ) x 5,
            'foo'
          ];
perl -MData::Dump::Streamer -le '@ary = (); x($ary[5],"foo"); sub x { 
+$_[0] }; Dump(\@ary)'
$ARRAY1 = [];
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As you can see, the element at index 5 is not created merely by passing its alias to the sub. It doesn't exist and isn't defined, and may remain so after the sub has been called.

_($_=" "x(1<<5)."?\n".q·/)Oo.  G°\        /
                              /\_¯/(q    /
----------------------------  \__(m.====·.(_("always off the crowd"))."·
");sub _{s./.($e="'Itrs `mnsgdq Gdbj O`qkdq")=~y/"-y/#-z/;$e.e && print}

You make a valid point(++), so I will modify my proposed behaviour statement to:

Elements of @_ are aliased to EXISTING or EXPLICITLY-passed element(s) of the passed parameter(s).
The sub may modify ONLY THESE elements. Attempts to modify othe elements will not be retained on sub exit.

     "An undefined problem has an infinite number of solutions." - Robert A. Humphrey         "If you're not part of the solution, you're part of the precipitate." - Henry J. Tillman

You still seem to be thinking you can pass an array. "Existing elements of the passed parameters" only makes sense if you think foo(@bar) in some way passes @bar to foo. And it doesn't; @bar expands to a list of its elements, with anonymous undefined values in place of nonexistent elements. But I don't know that this behavior is set in stone, so I'd be reluctant to see it documented.