The content of an array element has to be a reference
For example: $array[0]=\@otherarray
stores a reference to @otherarray into the first element
of @array.

In the second question, what you do first: @array=({hashkey => [@otherarray, @etc]})
stores into @array a reference to an anonymous
hash with just one key, hashkey, and a value, a reference
to an anonymous array (the joining of @otherarray and
@etc) and, again, the second line should use a reference: $array[0]{hashkey}[0]=\@otherarray;
perldoc perlref has a good tutorial on references; you
need to master them to do complex datastructures in Perl