run

(cat vars.erb template.erb) | ruby t.rb 

FOO = BAR
BAR = default bar

t.rb

require 'erb'

class Context
  def self.const_missing(name)
    nil
  end
  def binding
    super
  end
end

print ERB.new(STDIN.read).result(Context.new.binding)

template.erb

FOO = <%= FOO || 'default foo' %>
BAR = <%= BAR || 'default bar' %>

vars.erb

<%
  FOO="BAR"
%>