[nullymcnull]

There are some very good points here, but letting the junior go ahead and use Perl (or any language / platform that the company doesn't generally use or support, and that there's no overwhelming benefit to taking on for one small improvement), just to avoid discouraging them and helping them grow? Definitely not.

I do realize that part of becoming a senior engineer is being able to make and learn from mistakes like this - that sometimes loose/absent leadership is the crucible that makes them. But at this point the hypothetical senior is allowing unnecessary complexity into things, it's verging on outright dereliction of their bigger picture duty to keep things reasonably homogenous and maintainable. Mentoring juniors into seniors is something that a good senior engineer should spend quality time on, no doubt, but it's far from their primary function - and you certainly don't let your system descend into unmaintainable multiplatform anarchy for the sake of doing so in an optimally non-discouraging way. There's still always going to be a lot of blocking of dumb ideas - it comes with the territory, and not all ideas are salvageable - it's harsh but sometimes you need to hear that to grow, too.

Not every junior is a senior waiting to blossom, either - some are just solidly junior and not really equipped with the curiosity or drive to progress no matter what you do. Yes, there's room for bias to seep in here, but still, there's little sense in trying to make seniors out of devs who just aren't cut out for it (some of whom are still solidly dependable pairs of hands for day to day code slinging).