Perl comes with a whole bunch of cryptically-named built-invariables, which clever Perl programmers exploit to reduce the number oflines of code in their scripts. This article examines some of the morecommonly-used special variable in Perl, with examples and illustrations ofhow they may be used.
Perl comes with some very interesting variables specifically designed to store input arguments, for both scripts and subroutines. The first of these is the special @ARGV array, which contains a list of all the command-line arguments passed to the Perl program; each argument is indexed as an element of the array. Consider the following example, which demonstrates:
# get length of argument list
$num = @ARGV;
# iterate and print arguments
for ($x=0; $x<$num; $x++)
print "Argument " . ($x+1) . " is $ARGV[$x]\n";
Here's an example of the output (I called this script with the command-line arguments "red 5px Arial"):
Argument 1 is red Argument 2 is 5px Argument 3 is Arial
Perl also comes with a variable named @_, which contains arguments passed to a subroutine, and which is available to the subroutine when it is invoked. The value of each element of the array can be accessed using standard scalar notation - $_ for the first element, $_ for the second element, and so on.
In order to illustrate, consider the following example:
# define a subroutine
$sum = $_ + $_;
$total = &add_two_numbers(3,5);
print "The sum of the numbers is $total\n";
In the example above, once the &add_two_numbers subroutine is invoked with the numbers 3 and 5, the numbers are transferred to the @_ variable, and are then accessed using standard scalar notation within the subroutine. Once the addition has been performed, the result is returned to the main program, and displayed on the screen via the print() statement.