How to merge puppet array variables

As I recall you can't do it directly. Something like this might work though:

$default_env = ["a=b"]

Cron {
  environment => $default_env
}

class a {
  $additional_env = split(inline_template("<%= (default_env).join(',') %>"),',')

  cron {"test":
    command => "true",
    user => "me",
    environment => $additional_env
  }
}

include a

(the split/inline_template is based off of something from http://www.crobak.org/2011/02/two-puppet-tricks-combining-arrays-and-local-tests/ )