title: How can I check if an environmental variable is set?
body: <p>I have the following code in perl</p>
  
  <pre><code>my %Opt = 
    (
     boards_txt =&gt; "$ENV{'ARDUINO_DIR'}/hardware/arduino/boards.txt",
    );
  </code></pre>
  
  <p>In this you can see that the env variable <code>ARDUINO_DIR</code> is append. Some users might not have this variable set. If that is the case, then I want to hardcode a path.</p>
  
  <p>Question: How can I check if the env variable is set or not?</p>