Trivial fix.
To reproduce:
1. Create or edit a rule on /cgi-bin/koha/admin/smart-rules.pl - i.e. submit the form (make POST request)
2. Notice the mentioned above warning in plack-intranet-error.log
3. Apply the patch.
4. Repeat step 1.
5. That warning in log file should be no more.
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>