title: Error while using un-initialized variable in an expression in a template file
body: <p>I am trying to process a template file using template-toolkit version 2.29 and perl 5.28.1. I am getting below warning while processing template file <br/>
  <strong>Argument "" isn't numeric in addition (+) at  line xyz</strong> <br/> 
  I think it is because in the template file we have un-initialized variables which are used in an arithmetic expression. <br/> 
  <strong>Is it necessary to initialize a variable before using it in an expression if using template-toolkit version 2.29?</strong>
  In the below code the warning occurs for line 5.</p>
  
  <p>Below is the snippet from code. </p>
  
  <pre><code>[%- groups = {} -%] &lt;br/&gt;
  [%- cols = [ 10, 10, 20, 30 ] -%] &lt;br/&gt;
  [%- FOREACH c IN cols -%] &lt;br/&gt;
     [%- val = c -%] &lt;br/&gt;
     [%- groups.$val = groups.$val + 1 -%] &lt;br/&gt;
  [%- END -%] &lt;br/&gt;
  </code></pre>
  
  <p>If we initialize <code>groups.$val</code> before using it in the expression, there is no warning. <br/>
  <strong>I did not get the issue when I used template-toolkit version 2.15.
  Not sure what changed in 2.29 (or in any other version greater than 2.15) which is causing the warning?</strong></p>