How to share variables across my .rb files?

Constants (which include modules and classes) are added to the shared global environment:

phrogz$ cat constants1.rb 
TEST_VARIABLE = "test"

phrogz$ cat constants2.rb 
require_relative 'constants1'
p TEST_VARIABLE

phrogz$ ruby constants2.rb 
"test"

Instance variables declared in main are all part of the same main:

phrogz$ cat instance1.rb 
@test_variable = "test"

phrogz$ cat instance2.rb 
require_relative 'instance1'
p @test_variable

phrogz$ ruby instance2.rb 
"test"

Global variables are also all part of the same environment (tested in 1.8.6, 1.8.7, and 1.9.2):

phrogz$ cat global1.rb 
$test_variable = "test"

phrogz$ cat global2.rb 
require_relative 'global1'
p $test_variable, RUBY_DESCRIPTION

phrogz$ ruby global2.rb 
"test"
"ruby 1.9.2p180 (2011-02-18 revision 30909) [x86_64-darwin10.7.0]"