You can pass the Hash.new function a block that is executed to yield a default value in case the queried value doesn’t exist yet:
h = Hash.new { |h, k| h[k] = Hash.new }
Of course, this can be done recursively. There’s an article explaining the details.
For the sake of completeness, here’s the solution from the article for arbitrary depth hashes:
hash = Hash.new(&(p = lambda{|h, k| h[k] = Hash.new(&p)}))
The person to originally come up with this solution is Kent Sibilev.