Perl is the language I use most often for processing, with bash and PHP for redundancy and glue.

As others mentioned, I chose Perl and PHP because those are the languages I am most familiar with, and because they are "boring", meaning they've been stable enough that I could've written my scripts 20 years ago, and they would still work today. PHP to a lesser extent, but still true.

Also, as I mentioned the "lowest common denominator" style of writing allows me to write Perl which I can copy and paste into PHP almost without changes, and vice-versa. To facilitate this, I ported several functions from one to the other, e.g. str_replace for Perl and and index() for PHP.

I can't think of ways to make it more foolproof than writing two redundant systems, nor how much better tests I can have than full coverage of each process by a redundant one, except by introducing triple-redundancy, which is not out of the question. Is that what you had in mind, or something else?