How are you using Apache::AuthenNTLM? For this change to work you'll need to change any references to Apache::AuthenNTLM to use your new sub-class MyOverride. For example, if this was used as an Authen handler (just a guess) you would use:
PerlAuthenHandler MyOverride
Does that help?
BTW: the correct term for what you're doing above is overriding. Overloading refers to something totally different - see overload for details.