Tricks of the Wizards

`Filter::Util::Call`

        ...
        if ($status >= 0) { 
          if (/^\s*  no  \s+  Rot13  \s*  ; # "no Rot13;"
                \s* (?: #.* )?  $           # Optional WS or comment
              /x) {
            return $status;
          }
          tr/A-Za-z/N-ZA-Mn-za-m/;
        }
        ...

Note that this won't pick up a line like this one:

        no Rot13;   print "I like pie.\n";

But it will pick this up:

        $z = qq{
        no Rot13;
        };

You just have to hope that nothing like that comes along
In general, source filtering is based on hopes like this one
You can write a filter that works most of the time
But faced with sufficiently weird code, it will break