Probably the authors just meant "if you want to type in more members of the list," i.e. you won't have to add the comma to the previous line should you want to go on and add 4 to @numbers.

reference : p. 48 of 2nd edition of Programming Perl, this is my interpretation.

Update Grist for my interpretive mill: note the multiple kinds of efficiency discussed in Ch. 8, one of which is "Maintainer Efficiency", which applies to the present case quite nicely.

Second update : I don't proclaim guru-hood, but I'd suggest Effective Perl Programming, and second chromatic's suggestion about Conway's Object-Oriented Perl. You might also take a look at the Reviews section and see if any of the books there catch your interest.

Philosophy can be made out of anything. Or less -- Jerry A. Fodor