Local variables defined outside of a thread seem to be visible from inside so that the following two uses of Thread.new seem to be the same:
a = :foo
Thread.new{puts a} # => :foo
Thread.new(a){|a| puts a} # => :foo
The document gives the example:
arr = []
a, b, c = 1, 2, 3
Thread.new(a,b,c){|d, e, f| arr << d << e << f}.join
arr #=> [1, 2, 3]
but since a, b, c are visible from inside of the created thread, this should also be the same as:
arr = []
a, b, c = 1, 2, 3
Thread.new{d, e, f = a, b, c; arr << d << e << f}.join
arr #=> [1, 2, 3]
Is there any difference? When do you need to pass local variables as arguments to Thread.new?
When you pass a variable into a thread like that, then the thread makes a local copy of the variable and uses it, so modifications to it do not affect the variable outside of the thread you passed in
a = "foo"
Thread.new{ a = "new"}
p a # => "new"
Thread.new(a){|d| d = "old"}
p a # => "new"
p d # => undefined