perl – 为什么小循环中的大循环比大循环中的小循环更快？

$big=10_000_000;
#A:big loop outside
my $begin_time = time;
foreach my $i (1..$big) {
        foreach my $p (1..10){
        }
}
my $end_time = time;
my $t1=$end_time-$begin_time; 

#B:small loop outside
my $begin_time = time;
foreach my $i (1..10){
     foreach my $p (1..$big){
     }
}
my $end_time = time;
my $t2=$end_time-$begin_time; 

#output
print $t1;
print "n";
print $t2;

t1=8 seconds
t2=3 seconds

和mathematica代码：

Timing[Do[2,{i,1,10},{j,2*1,10^7}]]
output:{14.328,Null}
Timing[Do[2,2*10^7},10}]]
output:{30.937,Null}

为什么外面的大循环需要更多时间？