$ means you're working with one element.
@ means you're working with multiple elements.

@hash{@list}
is a hash slice. It amounts to
map { $hash{$_} } @list
but can be used as an lvalue.

$hash{@list}
is the same as
$hash{join($;, @list}}
It's a means of doing Hash of Hashes without actually using Hash of Hashes. It's generally unused.