There's nothing in your update that generates a warning either.

use strict;
use warnings;

BEGIN { package My::Guard; sub new {} }

{
   my $sth;
   my $guard1 = My::Guard->new(sub { $sth->finish });
   my $guard2 = My::Guard->new(sub { $guard1; });
}
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>perl script.pl

>
[download]

By the way, My::Guard already exists as ReleaseAction, Object::Destroyer, AtExit, Scope::Guard, Sub::ScopeFinalizer, etc.