Your call to
ChangeArray6(@A);
Passes the elements of @A into the subroutines @_.

Your implementation of "sub ChangeArray6" updates the contents of @_ which were previously aliased to the contents of @A.

Note: Only the contents of @_ were changed. The contents of @A were not.

Here are two alternative ways to update the contents of @A:

perl  -E '@a=(2)x2; @a=ch(@a);sub ch{@_=(6) x @_}; say for @a'
[download]

In the sample above, the change made to @_ is RETURNED, then ASSIGNED to @a, changing @a.

perl -E '@a=(2)x2; ch(\@a);sub ch{@{$_[0]}=(6) x scalar @{$_[0]}}; say
+ for @a'
[download]

In the second piece of code, we pass a REFERENCE to @a. The subroutine assigns new content to the thing(array ref-de-referenced) that it's first parameter referenced (@a).

See 'perldoc perlref'.