Short answer: Don't write $a->[0]->[0]->[0]->[0]->[0], write $a->[0][0][0][0][0]. It does the same thing.
Long answer:
As for writing code, however, you don't need to write $a->[0]->[0]->[0]->[0]->[0]. After the first dereference (if any), you can omit the '->'s because arrays and hashes only store scalars so perl knows you're talking about a reference. You can use all the arrows for backwards compatibility.
Perl 6 does not flatten by default. (And, obviously, sigils remain a core part of the language.