in reply to Using an undefined value as a hashref

Here is a trick to achieve your desired results at a cost of 5 more bytes ... 
(my $hashref={})->{'foo'} = 'bar';

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The initial assignment to {} could, of course, be replaced by an existing hash ref.

     "Man cannot live by bread alone...
         He'd better have some goat cheese and wine to go with it!"

In Section Seekers of Perl Wisdom