foo.clj

(ns test-app.foo
  (:require [clojure.java.io :as io])
  (:import [java.util Properties]))

;;dotenv (w/ precedence per Colin -- shell > .env.foo > .env)
(defn f->p
  [f]
  (let [file (io/as-file f)]
    (when (.exists file) (doto (Properties.) (.load (io/input-stream file))))))

(defn get-env
  []
  (let [env-vars  (System/getenv)
        dotenv    ".env"
        dotenvfoo (str dotenv "." (get env-vars "ENVIRONMENT"))]
    (->> [(f->p dotenv) (f->p dotenvfoo) env-vars]
         (map (partial into {}))
         (apply merge))))

;;environs
(defn get-var
  [m k]
  (or (get m k) (throw (ex-info (str "env var " k " not set") {}))))

;; app init
(swap! env (partial get-var (get-env))) ;; maybe alter a root def instead?

Use java.util.Properties to load the .env, .env.foo files since it knows how to parse name=val style properties expertly. merge things in the correct order of precedence, using nil where files don't exist.