It is important to note that Perl OO does not have any "built in" concept of data. There is no default Constructor and the Destructor consists of a call to DESTROY.

It might be more descriptive to rewrite Foo->new()->hello('test5'); as

bless(\sub {}, 'Foo')->hello('test5');
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This just blesses a CODE_REF for fun. It could be a HASHREF or some other REF. The only real difference between

'Foo'->hello()
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and

bless(sub {}, 'Foo')->hello()
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Besides the fact that the blessed version can contain data, is that just before the blessed object is Garbage Collected, DESTROY is called with it as the argument ($x->DESTROY). UNIVERSAL contains a DESTROY method, so it always exists.