This is fixed by commit 0b97d15690d19a1fee0c903da7f5898764d8f15e Author: Karl Williamson <khw@cpan.org> Date: Tue Mar 19 12:37:04 2019 -0600 PATCH: [perl #133937] Assertion failure This recently added assertion actually caught an error, which is a potential read beyond end of buffer. This doesn't actually happen in this case because this is a regular expression pattern, and the toker makes sure there is a trailing NUL (that isn't counted). The solution is to check the bounds before reading. -- Karl Williamson --- via perlbug: queue: perl5 status: open https://rt.perl.org/Ticket/Display.html?id=133937