Perl autovivifies (http://en.wikipedia.org/wiki/Autovivification) only when it has to, and when it has some idea of what value you want to implicitly create.
In your example, Perl can't create the '$self->{boo}' value implicitly because it has no idea what it should be. It would be inappropriate if it created it with 'undef', '""' or '0'.

  $thingy{'someElement'}++; 
  # vivifies it to '0' before you increment it to '1'

  $thingy{'someElement'} .= "Foo!";
  # vivifies it to the empty string...

  $thingy{'someElement'}{'otherElement'} = "Foo!";
  # vivifies someElement to a hash reference
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In your case, you would want Perl to create the element 'foo' so that it would exist in the structure of the object, but that would be detrimental in many cases, and make it difficult to check the structure of objects if every element was created when you looked at it.