Well, I did indeed spend some time playing with it before writing my comment. I first tried to compile the TypeScript project I'm working on, and it happens to be an Express server. After some minor unrelated fixes required (Perry does not understand importing "fs/promises", so I fixed it to import "fs" and then taking .promises) it said it needs JS runtime, and the smallest repro I found was

  $ cat index.ts
  import * as express from 'express';
  const app = express();

  $ perry index.ts
  Collecting modules...
    JS module: express -> /private/tmp/ex/node_modules/express/index.js
  Error: build pulled in `perry-jsruntime` (QuickJS-based eval-equivalent runtime)   via the following file(s):
    - /private/tmp/ex/node_modules/express/index.js [express]
  
  `perry-jsruntime` is treated as a privileged dependency on par with adding a JIT to the binary — it re-introduces arbitrary runtime code execution and defeats Perry's structural advantage over Node. Refusing to link by default. (#499)
  
  To enable, set `perry.allowJsRuntime: true` in the host package.json, or pass `--enable-js-runtime` on the CLI for a one-off build. (Falls under `--lockdown` deny set when that flag ships — see #496.)

Fair, but might have been worth including that in your initial comment because the docs don't mention that at all.