3.2. Special Array Indices
If you store in an array an element that is beyond the end of the array, the array is automatically extended as needed. There's no limit on its length as long as there's available memory for Perl to use. If Perl needs to create the intervening elements, it creates them as undef values.
$rocks = 'bedrock'; # One element... $rocks = 'slate'; # another... $rocks = 'lava'; # and another... $rocks = 'crushed rock'; # and another... $rocks = 'schist'; # now there are 95 undef elements
Sometimes you need to find out the last element index in an array. For the array of rocks that we've been using, the last element index is $#rocks.[*] That's not the same as the number of elements because there's an element number zero.
$end = $#rocks; # 99, which is the last element's index $number_of_rocks = $end + 1; # okay, but you'll see a better way later $rocks[ $#rocks ] = 'hard rock'; # the last rock
Using the $#name value as an index, like that last example, happens often enough that Larry has provided a shortcut: negative array indices count from the end of the array. But don't get the idea that these indices "wrap around." If you've got three elements in the array, the valid negative indices are -1 (the last element), -2 (the middle element), and -3 (the first element). In the real world, nobody seems to use any of these except -1, though.
$rocks[ -1 ] = 'hard rock'; # easier way to do that last example $dead_rock = $rocks[-100]; # gets 'bedrock' $rocks[ -200 ] = 'crystal'; # fatal error!