my @array = 1 .. 1_000_000; 
## Note the above doubles mem. consumption.
## Half of which is immediately availble to the rest of the program.

# Ripple the good values through the array
my( $s, $d );
for( $s = $d = 0; $s < @array; $d++, $s++ ) { 
 	$array[ $s ] %2 and $s++;;
      $array[ $d ] = $array[ $s ]; 
}; 
# and truncate it.
$#array = --$d;