cperl-mode causes emacs to lock up indefinitely.  I can't break the lock
except by killing the process.

Steps to reproduce:

1. Visit the attached file with cperl-mode.
2. Put the cursor at the beginning of line 44 ($$str =~ ...)
3. Type C-k (kill-line).  The line is not removed, and the cursor is
   stuck.  C-g doesn't help.

This is happening on 24.3 and on trunk.