Some languages make monkeypatching a lot less dangerous.

For eg. in Perl you can use dynamic scoping to localise its effect:

  {
    no warnings 'redefine';
    local *SomeModule::some_func = sub { say "MONKEYPATCHED!" };
    
    # now everything in this scope that uses or calls SomeModule->some_func 
    # will now use the monkeypatched version
  }

  # where has everything else outside this scope remains unaffected

In Ruby Refinements earmarked for ruby 2.0 will have something similar: http://www.rubyinside.com/ruby-refinements-an-overview-of-a-...