Actually, a "sub", short for subroutine, is nothing but a function that doesn't have any parameters. Putting
sub Stuff{
do stuff;
do more stuff;
do even more stuff;
}
somewhere in your script lets you type Stuff; to do all the stuff specified before, Stuff being your subroutine here.
That is just terminology, however. You can include code stored in another file using the #include_function command just fine, it just isn't called a "sub". :V
Sorry for nitpicking, but if we don't keep the terminology clear, confusion will arise sooner or later.