Since hashes aren't inherently ordered, there isn't a notion of appending. Ruby hashes since 1.9 maintain insertion order, however. Here are the ways to add new key/value pairs.
The simplest solution is
h[:key] = "bar"
If you want a method, use store:
h.store(:key, "bar")
If you really, really want to use a "shovel" operator (<<), it is actually appending to the value of the hash as an array, and you must specify the key:
h[:key] << "bar"
The above only works when the key exists. To append a new key, you have to initialize the hash with a default value, which you can do like this:
h = Hash.new {|h, k| h[k] = ''}
h[:key] << "bar"
You may be tempted to monkey patch Hash to include a shovel operator that works in the way you've written:
class Hash
def <<(k,v)
self.store(k,v)
end
end
However, this doesn't inherit the "syntactic sugar" applied to the shovel operator in other contexts:
h << :key, "bar" #doesn't work
h.<< :key, "bar" #works