I am disturbed by the fact that "extra fun" does not make into the Dumper output

I'm pretty sure that's because my $x->{a}->[0] = "no you don't!"; is mostly equivalent to my $x; $x->{a}->[0] = "no you don't!";. From Private Variables via my():

All listed elements must be legal lvalues. ... The my is simply a modifier on something you might assign to.