Referencing a global hash from Puppet DSL

I'm trying to reference a global hash from a Puppet DSL manifest, but scope.lookupvar is failing to find it.

My declaration is in rubydefs.pp and is of the form

$my_hash = JSON.parse(`...some stuff that spits out JSON...`)

I use this hash in my templates with no trouble.

In my DSL file mymodule.rb I try to reference the hash like this:

foo = scope.lookupvar('::my_hash')
foo.keys.each do |i|
...

but foo always ends up undefined. I've tried just 'my_hash' (without the ::) with the same result. This is the error message from the puppet agent:

err: Could not retrieve catalog from remote server: Error 400 on SERVER: undefined method `keys' for :undefined:Symbol ....

I'm definitely including the files in the right order; my site.pp looks like

import "rubydefs.pp"
...
node mynode {
    include mymodule
}

How can I reference this hash from a Puppet DSL file?