Ruby arguments evaluated on function call. So if you have default arguments like this a = {} it means Ruby will create new object for every call without arguments. Lets see in action:
Example 1:
$ irb
> def test_hash(a={}); end
v1 = ObjectSpace.count_objects
10000.times{ test_hash }
v2 = ObjectSpace.count_objects
v2[:T_HASH] - v1[:T_HASH]
=> 10015
exit
In first example Ruby created 10000 anonymous hashes which will be sweeped on the next GC run. But do you want to make a job for it in first place? And then they complain about GC pauses :/
Example 2:
$ irb
> def test_nil(a=nil); end
v1 = ObjectSpace.count_objects
10000.times{ test_nil }
v2 = ObjectSpace.count_objects
v2[:T_HASH] - v1[:T_HASH]
=> 18
exit
$ irb
> EM_HASH = {}.freeze
def test_hash2(a=EM_HASH); end
v1 = ObjectSpace.count_objects
10000.times{ test_hash2 }
v2 = ObjectSpace.count_objects
v2[:T_HASH] - v1[:T_HASH]
=> 19
exit
No anonymous hashes.