If you forget the 1 at the end of a package, Perl tells you "The package didn't return a true value". Well, if it knows you forgot it, why not just put it there for you?
Because Perl modules are required to return a value to signal if the require directive must succeed (true value returned) or fail (false value returned; this can make sense if the module failed to initialize for some reason).
If you don't return anything, the interpreter cannot know if the require must succeed or fail; at the same time, since it's easy to forget to put the true value at the end of the package, it suggests the "common fix" for this error: add a true value as a return.
For some other info/folklore about the modules return value have a look at this question.