Perl 5 doesn't really make much of a distinction between methods and functions. You can call a function as a method and a method as a function. There are two important differences:

• methods are looked up and potentially inherited
• methods have the invocant passed as the first parameter

That's why Test::MockObject works -- you can call a method with the function semantics and manually pass an invocant:

In your case, you're just having Perl look for Add_Delta_Days in Date::Calc or a parent class, call the nearest sub it finds, and pass 'Date::Calc' as the first argument. Since the module never planned for that, it'll pull the string into $year and that's when things break.

No inheritance lookup, no invocant passing.