$x->{a} = 1 # 1 for $x->{a}; () # '', undef, { a => 1 } # +fetch
$x->{a} = 1 # 1 for $x->{b}; () # '', undef, { a => 1, b => undef } # +fetch
$x->{a} = 1 # 1 for $x->{a}; () # '', undef, { a => 1 } # +fetch
$x->{a} = 1 # 1 for $x->{b}; () # '', undef, { a => 1, b => undef } # +fetch