I would state it slightly differently. The function returns nothing different when it is passed no parameters because it takes no parameters — or more precisely, it does nothing with any parameters it may be passed. The array  @list_of_numbers is accessed (not declared) within the function. This treats the array as a global variable (even though technically it is lexical). I notice other instances of such access in your functions. Accessing a mutable global (as opposed to a constant) within a function is generally considered a Bad Thing, and is officially Frowned Upon. You have been warned.

Which is why when I add use strict; it gives me errors? How would I go about making my variables constant rather than mutable global within a function? I would prefer not to be entangled with Bad Things or Frowned Upon.