In your first example you're essentially copying the reference. You probably want to do something like this

$this = { foo => bar };
$that = { %$this };
$that->{foo} = "not bar";
print $this->{foo};
__END__
bar
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This works because it creates a new anonymous hash and assigns it to $this, whereas in your first example you're just dereferencing $this and then assigning a reference to it (still with me ;-). So in your second example you're creating a new hash in %that and then assigning a reference to it, which is why it doesn't effect $this.
HTH

broquaint