Using BasicObject

erb_with_hash.rb

module ERBWithHash
  class HashAsBinding < BasicObject
    def initialize(hash)
      @_env = hash
    end
    
    def method_missing(msg, *args, &block)
      super if block
      super unless args.empty?
      @_env.fetch(msg) { @_env.fetch(msg.to_s) { super }}
    end
  end

  def result_with_hash(hash)
    b = HashAsBinding.new(hash).instance_eval {Kernel.binding}
    result(b)
  end
end

if __FILE__ == $0
  require 'erb'
  require 'set'

  class ERB
    include ERBWithHash
  end

  my_erb = ERB.new(<<EOS)
file_size: <%= x.read.size%>
collection:
<% z.each_with_index do |it, idx| %> [<%= idx%>] <%= it %>
<% end %>

EOS

  puts my_erb.result_with_hash(x: File.open(__FILE__), y: 3, z: ['z', 'zz'])
  
  set = Set.new
  set << 'a'
  set << 3
  set << Thread.current
  
  puts my_erb.result_with_hash(x: File.open(__FILE__), y: nil, z: set)
end