CHI requires a switch to keep track of cache size. Also, flush should be mapped to CHI's clear method. Fixes bug #11565.

docs/changelog/7.x.x.txt

@@ -10,6 +10,7 @@
  - fixed #11533: Saving entered data at registration
  - fixed #11564: Shop: Cart does not require a state field in the address
  - fixed #11562: Missing perl modules in gotcha
+ - fixed #11565: Can't clear cache on plainblack.com
 
 7.9.4
  - We're shipping underscore.js now for its suite of extremely handy utility

lib/WebGUI/Cache/CHI.pm

@@ -59,7 +59,7 @@ Delete the entire cache namespace
 
 sub flush {
     my ( $self ) = @_;
-    $self->{_chi}->purge;
+    $self->{_chi}->clear;
 }
 
 #----------------------------------------------------------------------------
@@ -93,7 +93,8 @@ sub new {
     my $chi;
     unless ( $chi = $session->stow->get( "CHI" ) ) {
         my $cacheConf    = $session->config->get('cache');
-        $cacheConf->{namespace} = $namespace;
+        $cacheConf->{namespace}     = $namespace;
+        $cacheConf->{is_size_aware} = 1;
 
         # Default values
         my $resolveConf = sub {