Autovivification doesn't happen in this case, e.g.

my $foo = [qw(a b c)];
if (defined $foo->[4]) {
    print "this will not print\n";
}
print join('|', @$foo), "\n";
[download]

... prints "a|b|c" instead of "a|b|c||" as you would expect if autovivification had occurred.