use Benchmark qw(:all);
my ($resultPre, $resultNormal);
timethese(-100, {
    preAssign => sub {
        my %h;
        keys %h = 1e8;
        $h{$_} = $_ for 1..1e8;
        $resultPre = scalar %h unless $resultPre;
    },
    normal => sub {
        my %h;
        $h{$_} = $_ for 1..1e8;
        $resultNormal = scalar %h unless $resultNormal;
    },
});

print "Pre: $resultPre\n";
print "Normal: $resultNormal\n";