title: Why are Perl source filters bad and when is it OK to use them?
body: <p>It is "common knowledge" that <a href="http://perldoc.perl.org/perlfilter.html" rel="nofollow noreferrer">source filters</a> are bad and should not be used in production code.</p>
  
  <p>When answering a <a href="https://stackoverflow.com/questions/1785556/what-concerns-should-i-have-if-i-use-smartcomments-in-development-code">a similar, but more specific question</a> I couldn't find any good references that explain clearly why filters are bad and when they can be safely used.  I think now is time to create one.</p>
  
  <ol>
  <li>Why are source filters bad?</li>
  <li>When is it OK to use a source filter?</li>
  </ol>