You are right! Now that I was able to automate the testing I'm seeing clearer.

The bug is not present with emacs-24 -q but with emacs-27 -q

For the unwary, -q means that I don't load my personal config so that I can test vanilla emacs and not my own fixes, patches and cperl-modes.

What I forgot tho was explicitly switching to Ilya's cperl-mode, gnu-emacs is favorizing it's own perl-mode, which is much simpler and probably doesn't even try to handle y

Now running this (windows version)

c:\nonBKU\emacs-27.1-i686\bin>runemacs.exe -Q D:\tmp\pm\tst.pl -eval "
+(progn (cperl-mode)(goto-line 4)(newline))"

c:\nonBKU\emacs-27.1-i686\bin>
[download]

will show the bug and an error message

End of ‘y_ ... _’ string/RE not found: (scan-error Unbalanced parentheses 49 80)

That's the content of tst.pl:

#!/usr/bin/perl
use strict;
use warnings;

sub y_max {
    1
}

print "Done";
[download]

Automated tests are cool! xD

update

> there's a patch which you should be able to use at least in Emacs 27.1.

it worked in 24.3 so something must have introduced the bug later.