undef $bar may have some useful side-effects, if $bar is an object.

$bar = Foo->new();
...
undef $bar;
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will invoke "Foo::DESTROY", if it is defined.

$bar = 0;
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will also invoke DESTROY, but not if '0' is a valid assignment in the "Foo" class (long shot, I know).

Originally posted as a Categorized Answer.