How to reproduce:
In a buffer, activate cperl-mode (M-x cperl-mode), then enter:

$a++ / $b;

This is supposed to be a division.  However, cperl-mode interprets the
slash as the start of a regular expression.  Accordingly, the following
text is displayed with the wrong face, and also cperl-mode might
complain that it can't find the end of the regular expression.

Emacs does not crash.

Attached: A patch which fixes the issue for $a++ and $a--, and also
includes a test to verify that the very similar text "$a+ / $b" is still
correctly interpreted as a regular expression.

Disclaimer: I'm rather new to elisp and even newer to ERT.

Cheers,
haj