I think that the OPs problem is that an array-type variable, @a, is being assigned an array reference, $href.

Sorry, but that's not the right way to look at it. @a is not being assigned a reference, instead, the @{...} dereferences the array reference, and it can be used in place of any normal array (perlref), so in effect my @x = @{...} is like assigning an array to an array.

In your third one-liner example, $a plausibly could be undefined.

I'm not sure what you mean, $a is initially undefined in every example. Autovivification is what is causing both $a to go from being uninitialized to holding a hashref (because it's being dereferenced as such with ->{...}) and $a->{list} to come into existence as a hash entry with a value of [] (empty array ref). The third example does not work because in my @x = @{$a->{list}};, the @{$a->{list}} is not in lvalue context. Personally, I think in this case the autovivification behavior is fairly consistent with DWIM:

• In @x = @{$a->{list}}, I am asking Perl to access and dereference a reference, but there is no such reference there - the thing I expected to be there isn't, so Perl complains.
• In @{$a->{list}} = (), I am explicitly telling Perl that I want to assign something to that reference and I want it to be an array reference, so it makes sense for that thing to come into existence.
• The only "potentially surprising" thing here is the foreach behavior, which I explained in the first reply.

Edit: Just moved a sentence so the order makes more sense.

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