Oh, I have no idea at all beyond what's in that talk and the README from that repo.

I still use SciTE which can handle api files like the perl.api file now hosted in this repo. Turned it back on really quick and it looks like this: https://imgur.com/a/jI7IvMD which is almost exactly what you're asking for but SciTE is hardly an environment designed to rival "Komodo, IntelliJ, VS Code, Padre, Emacs, VIM" or anything that aims to be a full IDE.

Addendum: Notepad++ and Padre (edit: and apparently Komodo) are based on Scintilla. SciTE was designed as a testbed for the Scintilla editing component way back in the late 90s and has kept up with the component's changes. I'd use Notepad++ but I no longer work on Windows and it doesn't work on Linux or ChromeOS.