Why did such an experienced leader make such an terrible mistake?

The promises made didn't hold the reality delivered.

• switching to Perl would unlock the architecture we need, rebuilding from scratch would accelerate hiring and quality)
• velocity collapsed as the team relearned and rebuilt everything, burn rate jumped from 200K to 500K per month

One strategy: how you evaluated other language candidates?

Every technical debate is really two conversations: the visible and marketed one and the (stronger private) invisible one.

The real question is: can you afford to let it make your decisions?

Because the invisible conversation has a price tag. Industry research suggests that technology stack decisions account for 40-60% of total development costs over a product’s lifecycle. Research by Stripe found that developers spend 42% of their time on technical debt.

An better question is "what is this language going to cost us?" in velocity, technical debt, hiring difficulty, operational complexity.

spf13 published a framework to estimate it: https://spf13.com/p/the-9-factors/