I had the same question on perl-language list, and here is theDamian answer :

> Is the following correct for := :
>
> left side is like function in the respect that the right side is treated
> differently depending on the left-side-types i.e. left side is like
> prototype!! for the right side.
Yep.
> (@a ,@b) := (@b,@a)
> is the same as :
> (\@a, \@b) = (\@b, \@a);#if we had ref-allowed on the
left in perl5 of > cource :")

Yep.

> ($x,@y) := (@b, @a)
> is the same as :
> ($x, \@y) = ($b[0],\@a);

Nope. The $x lvalue confers scalar context on the first rvalue, so it's the same as:

($x,\@y) := (\@b, \@a)

> Which is most close explanation : BIND or ALIAS

It's a bind *and* an alias! ;-)