use Benchmark::Timer;

$T = new Benchmark::Timer;

$T->start( 'autoviv' ); 
$auto[ $_ ] = 1 for map{ 10**$_ } 0 .. 7; 
$T->stop( 'autoviv' ); 

$#pre = 10**7; 
$T->start( 'prealloc' ); 
$pre[ $_ ] = 1 for map{ 10**$_ } 0 .. 7; 
$T->stop( 'prealloc' ); 
$T->report;

1 trial of autoviv (93.750ms total)

1 trial of prealloc (15.625ms total)