I've had enough problems with code with global effects both causing bugs in our code and having performance hits that we've today developed a system to handle local CPAN patches to make these issues go away. One thing I did to check to see how widespread the problem is to run the following code over our CPAN library:
ack '(?:CORE::GLOBAL|UNIVERSAL)::[[:word:]].*=' lib/
The results were depressing and I'll be outlining our 'local patching' system in the test talk I'll be giving in Oslo (and hopefully at OSCON).