Well then, if the line shouldn't actually be commented out, then the problem is as talexb described, "one of the values in the array that's being sorted is undefined." You have two ways of solving that problem:

• Use an empty string value instead of undef
• Turn off the warning

Which solution you pick will usually depend highly on the application. I tend to structure my apps in a way that an empty string makes more sense than undef, but perhaps this one isn't. In that case, turning off the warning (using a lexically scoped no warnings) would be the best option.