$rocks[0] = 'bedrock';
$rocks[1] = 'slate';
$rocks[2]= 'lava';
$rocks[3] = 'crushed rock';
$rocks[99] = 'schist';

$#rocks = 2; # forget all rocks after 'lava'
$#rocks = 99; # add 97 undef elements (the forgotten rocks are gone forever)