with_var.rb

def with_var(variable, value, &block)
  old_value = eval variable
  eval "#{variable} = #{value.inspect}"
  ret = block.call
  eval "#{variable} = #{old_value.inspect}"
  ret
end

# Usage
# =====

puts "Foo starts at #{ENV['FOO']}"

with_var "ENV['FOO']", "33" do
  puts "Foo is now #{ENV['FOO']}"
end

puts "Foo ends at #{ENV['FOO']}"

Be sure not to run with_var("FileUtils.rm_rf('/bad'); ENV['FOO']", 33) !