The @external_resource module attribute is a good place to start:

https://hexdocs.pm/elixir/Module.html#module-external_resource

You could point that at your XML file, then parse it into a module attribute:

defmodule BagOfXml do
  @external_resource "path/to/some.xml"

  @parsed_data SomeOtherModule.parse("path/to/some.xml")

  def lookup_thing(id) do
    # use @parsed_data and id to do stuff
  end
end

This will call SomeOtherModule.parse at compile-time (it cannot be a function in the same file) and store the resulting data in the compiled BEAM file.

@external_resource will ensure that if the XML file changes the module will be recompiled.