The Invocation Arguments
Technically, the invocation diamond operator isn’t literally looking at the invocation
arguments—it works from the @ARGV array. This array
is a special array that is preset by the Perl interpreter as the list of
the invocation arguments. In other words, this is just like any other
array, (except for its funny, all-caps name), but when your program
starts, @ARGV is already stuffed full
of the list of invocation arguments.[†]
You can use @ARGV just like any
other array; you could shift items
off of it, perhaps, or use foreach to
iterate over it. You could even check to see if any arguments start with
a hyphen, so that you could process them as invocation options (like
Perl does with its own -w
option).[‡]
The diamond operator looks in @ARGV to determine what filenames it should
use. If it finds an empty list, it uses the standard input stream;
otherwise it uses the list of files that it finds. This means that after
your program starts and before you start using the diamond, you’ve got a
chance to tinker with @ARGV. For
example, here we can process three specific files, regardless of what
the user chose on the command line:
@ARGV = qw# larry moe curly #; # force these three files to be read
while (<>) {
chomp;
print "It was $_ that I saw in some stooge-like file!\n";
}[†] C programmers may be wondering about argc (there isn’t one in Perl), and what
happened to the program’s own name (that’s found in Perl’s special
variable $0, not @ARGV). Also, depending upon ...