What makes you think that we have both functions and subroutines in Perl? Actually we only have subroutines.

Generally in computer science a function is a special type of subroutine that returns a values (as opposed to being called just for its side-effects). But in Perl (as the cookbook says) we don't make that distinction.

As an aside, could you please refrain from posting links to copyright-infringing web sites. O'Reilly's authors worked hard to produce those books - it would be nice if they were compensated for their effort.