Proc wrapped eval, eval only once...

proc_wrapped_eval.rb

require 'benchmark'

class ProcWrappedEval
  attr_accessor :callable

  def initialize(code)
    @callable = eval("::Proc.new { #{code} }")
  end
end

iterations = 1000000
test_code = "6 * 7"
callable = ProcWrappedEval.new(test_code).callable

Benchmark.bmbm do |x|
  x.report("proc wrapped eval:") do
    iterations.times { instance_eval(&callable) }
  end

  x.report("instance_eval:") do 
    iterations.times { instance_eval test_code }
  end
end

Results

Rehearsal ------------------------------------------------------
proc wrapped eval:   1.020000   0.030000   1.050000 (  0.605000)
instance_eval:       2.950000   0.030000   2.980000 (  2.273000)
--------------------------------------------- total: 4.030000sec

                         user     system      total        real
proc wrapped eval:   0.200000   0.000000   0.200000 (  0.204000)
instance_eval:       1.800000   0.010000   1.810000 (  1.767000)

Results:

Rehearsal ------------------------------------------------------
proc wrapped eval:   1.080000   0.020000   1.100000 (  0.571000)
eval:                3.370000   0.030000   3.400000 (  2.363000)
--------------------------------------------- total: 4.500000sec

                         user     system      total        real
proc wrapped eval:   0.200000   0.000000   0.200000 (  0.199000)
eval:                1.800000   0.000000   1.800000 (  1.756000)