Add @_ in signatured sub being experimental to perldelta.pod

pod/perldelta.pod

@@ -27,6 +27,34 @@ here, but most should go in the L</Performance Enhancements> section.
 
 [ List each enhancement as a =head2 entry ]
 
+=head2 @_ is now experimental within signatured subs
+
+Even though subroutine signatures currently remain experimental, use of the
+default arguments array (C<@_>) with a subroutine that has a signature is
+specifically also experimental, with its own warning category.  Silencing the
+C<experimental::signatures> warning category is not sufficient to dismiss
+this.  The new warning is emitted with the category name
+C<experimental::args_array_with_signatures>.
+
+Any subroutine that has a signature and tries to make use of the defaults
+argument array or an element thereof (C<@_> or C<$_[INDEX]>), either
+explicitly or implicitly (such as C<shift> or C<pop> with no argument) will
+provoke a warning at compile-time:
+
+    use experimental 'signatures';
+
+    sub f ($x, $y = 123) {
+      say "The first argument is $_[0]";
+    }
+
+Z<>
+
+    Use of @_ in array element with signatured subroutine is experimental
+    at file.pl line 4.
+
+The behaviour of code which attempts to do this is no longer specified, and
+may be subject to change in a future version.
+
 =head1 Security
 
 XXX Any security-related notices go here.  In particular, any security