$#a is the last index of the array @a. setting it to something lower than what it already is will truncate the array.

@a = (0..10);
print join(" ", @a), "\n";
$#a = 5;
print join(" ", @a), "\n";
[download]

0 1 2 3 4 5 6 7 8 9 10
0 1 2 3 4 5