IMO the "Perfect Solution" is to have the app receive its mount prefix from the reverse proxy, and then you can even mount the app at different host/paths simultaneously. In other words, each request supplies the prefix to be used for that request.

I don't use Dancer, and I try to avoid Apache, but the checklist looks something like:

• Check your HTTP headers. The request path should have the prefix removed. There should be another header like X-Forwarded-Prefix (the default in Traefik) that should specify the prefix. If not, fix the config in the reverse proxy.
• Check your Plack environment. It should have the prefix in SCRIPT_NAME, and not in PATH_INFO. If not, fix your middleware until it does.
• When generating URLs in templates, always include the prefix, which your framework should provide in some convenient variable. Never include the hostname or protocol. If your framework insists on that, modify it until it doesn't.
• When returning redirects, find out whether your framework automatically adds the prefix, or if you need to add it manually. Never include the protocol or host name in the redirect URL (even though that's mandated by the standard) and leave that task to your reverse proxy. (path-only 3xx locations work in every browser since forever, and proxies will usually rewrite them to full URLs for you anyway)

There are many ways to do it of course, including Apache mod_proxy_html that can rewrite your html output to modify the generated URLs, but this just leads to frustration. (If you're not in control of the application, sometimes its the only option, though)