G'day slackcub,

Welcome to the monastery.

I don't own that book; however, for those that do, it would have been helpful to identify where in the book that one line of code exists.

I expect that the book is talking about reading the contents of files whose filenames occur on the command-line. Given this file:

$ cat pm_1049671_temp.txt
a b c
d e f
g h i
[download]

Here's what I think the book is trying to explain:

$ perl -Mstrict -Mwarnings -le '
    my @numbers;
    push @numbers, split while <>;
    print "@numbers";
' pm_1049671_temp.txt
a b c d e f g h i
[download]

You can have more than one filename on the command-line. Here's another file:

$ cat pm_1049671_2temp.txt
j k l
m n o
p q r
[download]

Here's the same code reading the contents of both files:

$ perl -Mstrict -Mwarnings -le '
    my @numbers;
    push @numbers, split while <>;
    print "@numbers";
' pm_1049671_temp.txt pm_1049671_2temp.txt
a b c d e f g h i j k l m n o p q r
[download]

The actual command-line arguments are available to your program in the @ARGV array:

$ perl -Mstrict -Mwarnings -le '
    print "@ARGV";   
' pm_1049671_temp.txt pm_1049671_2temp.txt
pm_1049671_temp.txt pm_1049671_2temp.txt
[download]

Here's some further reading that may be helpful to you:

• perlop: I/O Operators — I suggest you read the introductory paragraphs (for background information) and then, a little further down, you'll find "The null filehandle <> is special: ..." which relates directly to your current issue.
• perlvar: Variables related to filehandles — Again, read the introductory information then, further down, find $ARGV, @ARGV, ARGV and ARGVOUT; $. (a little after those) also has relevant information.
• perlrun — This has useful information regarding command-line usage; search for "<>" and "ARGV" to find items related to your question. There's information about other ways to deal with filenames on the command-line: I'll leave you to reseach this yourself.