I remember Ilya struggling and swearing about the way the emacs maintainers patched code and broke backwards compatibility.

Yes, that was the case for some time, leading to Emacs maintainers not wanting to touch bugs in CPerl mode. Back then Ilya maintained his own version for downloading, and Emacs maintainers occasionally applied his changes with new Emacs releases. This continued until 2009. But Ilya's archive is gone by now, so there's no longer an "upstream" for the bundled version.

Five years ago Ilya announced that he could not touch for CPerl mode for years, and he would let us know if/when he's back. This has not happened until today.

Since then, there was a CPerl version bundled with Emacs, and for this bundled version there was no particular reason to keep backwards compatibility. So, the Emacs maintainers eliminated workarounds for Emacs 19, XEmacs specifics, and also functions which were deprecated in Emacs. This continues until today.

As I found out, the Emacs maintainers these days review and accept patches for CPerl mode, so I started providing them last year. At first, working through the list of known bugs, and then adding new stuff. Also among these patches were backports for some functions so that this bundled version of CPerl mode now works with Emacs 26.1 and above.

So, practically it means that M-x report-emacs-bug is the best way to report bugs in CPerl mode. On the plus side, these bug reports get more attention today than they did ten years ago.