How about C if you dont already know it. Then you could contribute to the Perl 5 core and have your code used all the time throughout the world.
That's what I did. After 8 years of retirement, but maintaining a small non-profit's website using perl to automate things, I discovered a bug in perl that was causing me a lot of problems. I had filed a bug report with Perl while I was still $working, and it got fixed pretty promptly. But this time nothing happened. I didn't know Perl was open source, but when I discovered that, I volunteered to fix this bug so I could get back to the website maintenance. I found my mind was rusty. The bug was much more involved than I had expected, but after a couple of months of working on it, I realized that my mind was less rusty. And so I've been doing it ever since. And the core needs more people to help maintain it. And you'd stay relevant as you age. The song "Irrelevance" by Susan Werner pretty much sums it up: https://www.youtube.com/watch?v=bRW1SuInEgA