And, frankly, being able to depend on timely destruction would be nice.

It's a Perl5 feature! Don't know about Perl6.

Destruction of SVs is not related to what has a hard reference (C pointer) to it. As soon all the lexical variables, package variables, stack reference, etc disappear, destruction occurs (including calling the appropriate DESTROY).

use Devel::Peek;

sub DESTROY { print STDERR "*Destroyed*\n" }

{
   local $foo;
   {
      my $bar = bless {};
      $foo = $bar;
      Dump($foo);
      print STDERR ("Exiting inner\n");
   }
   Dump($foo);
   print STDERR ("Exiting outer\n");
}

print STDERR ("Exiting file scope\n");
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Here's the relevant lines of the output:

    REFCNT = 2
Exiting inner
    REFCNT = 1
Exiting outer
*Destroyed*
Exiting file scope
[download]

Re^2: deleteing references might help. It doesn't matter whether a value is referenced by a lexical var, by a package var, or by the underlying memory structure of an array. As soon as something's refcount reaches zero, it is destroyed. In the figure, "Automatically" can be read as "Immediately".

Calling of DESTROY is neither timely nor ordered.

You're thinking of global destruction. Scope-exiting destruction in Perl 5 is indeed timely and deterministic (though you probably shouldn't rely on the order of destruction of $foo, $bar, and $baz in the following example):

{
    my $foo = 1;
    my $bar = 2;
    my $baz = 3;
}
[download]

Localized restoration uses the same reference-counting mechanism, so the appropriate destruction there is timely.