Perl did have `use strict`, so there was at least some plausible path to making non-breaking changes under a new pragma.

The OP’s theory that Perl 6’s radicalism allowed Perl 5 to be conservative sounds right to me.

Though at the same time the bit where `use strict` was optional wound up being off-putting to a lot of us, at least in part because we'd always wind up with _something_ that wasn't designed for `use strict` and had, uh, interesting failure modes.

It's the same drive that we see from JS to TS these days, or adding type hints to Python, and even to some extent why people pick up Rust: because you get a refusal to act and an explanation rather than wonky results when you goof.

IME there's been a wider shift away from worse-is-better, and Perl was kind of one of the early casualties of that. Part of that is also how science has marched on: When Python and Perl were new, the most popular typed languages were kind of tedious but not what people would consider _good_ at types these days. Perl was the first language I learned, and if I was transported back to the 1990s, I'd probably still pick it, even if I don't use it in 2025.

(OK, maybe I'd go all in on OCaml. Any way the camel wins.)