Cperl-mode syntax highlighting breaks when inserting a newline into a here doc

Question

in cperl-mode, here docs are colored with a font-lock-string-face. However if you used any function that inserts a newline on a blank line (e.g. M-: (insert "\n")) it will cause all code after the newline to be highlighted with font-lock-comment-face. The only way to fix it is try and "mess with" here doc tags so that cperl re-parses and corrects the font-lock. I have a feeling this may have something to do with this issue since it is also dealing with multi-line syntax highlighting. Is there an expert out there who can help me find a way to fix this?

#!/usr/bin/env perl

my $here_doc = <<'_HERE_';
this is here doc line
<--- call (insert "\n") when cursor is here
_HERE_

Answer 1

Eventually this bug has been fixed in the Emacs repository. You can use cperl-mode.el directly from that repository with Emacs 26.1 or newer.

The fix does what could have been done in font-lock-extend-region-functions as suggested by wasamasa and now uses the same method as perl-mode.el to mark here-documents.

Answer 2

The package highlight-refontification visualizes how font-lock refontifies a buffer. In this case, it starts at the line where the point is.

One way to handle this is to add a function to font-lock-extend-region-functions to expand the region to include the full heredoc comment.

One thing that is curious is that the highlighting becomes correct when pressing return but not when evaluating (insert "\n"), something I have no explanation for.