($a,undef,$b)= @x;    # [undef] is copied
($a,"hi",$b)= @x;     # "hi" is aliased and so this dies
@x= ($a,undef,$b);    # [undef] is copied so $x[1] is modifiable
@x= ($a,"hi",$b);     # "hi" is copied so $x[1] is modifiable