In a previous blog post I wrote that is not possible to completely turn off the Query Cache in MySQL before 5.5 .
Although, it is possible to completely turn off the Query Cache with Percona Server 5.1 .
MySQL bug #38551 reports that the change is incompatible with MySQL 5.1 . In this case the incompatibily is only related to how the server behaves: if Query Cache is disabled at startup it cannot be enable at runtime.
Thanks to Kenny Gryp from Percona for pointing this out and for the clarifications provided.