Why are Perl source filters bad and when is it OK to use them?

Question

It is \"common knowledge\" that source filters are bad and should not be used in production code.

When answering a a similar, but more specific question I couldn\'t

Answer 1

I don't like source filters because you can't tell what code is going to do just by reading it. Additionally, things that look like they aren't executable, such as comments, might magically be executable with the filter. You (or more likely your coworkers) could delete what you think isn't important and break things.

Having said that, if you are implementing your own little language that you want to turn into Perl, source filters might be the right tool. However, just don't call it Perl. :)