Your last example is comparable to

perl -wMstrict -MData::Dump -e'  my $x =  $a->{list}->[0];  dd $a'
{ list => [] }
[download]

since indexing autovivifies the anonymous array, as perlref explains. Retrieving its highest index is also a way of indexing.

But I agree, autovivification is important enough to warrant a section on its own in e.g. perldata, which mentions it only once and refers to perlref.