If I run this:

perl -MTime::HiRes=time -wE"my $t=time; my @a=1..1e6;say time-$t; <>"
0.0623979568481445
[download]

It takes 6/100ths sec; consumes 84MB and causes 24,000 page faults.

This:

perl -MTime::HiRes=time -wE"my $t=time; my @a=map $_,1..1e6;say time-$
+t; <>"
0.216002941131592
[download]

2/10ths ec; 108MB; 30,000 page faults.

But this:

perl -MTime::HiRes=time -wE"my $t=time; my @a=map $_,map $_,1..1e6;say
+ time-$t; <>"
31.4807999134064
[download]

32 seconds; 140MB; 5.7 million page faults?

But most confusingly:

perl -MTime::HiRes=time -wE"my $t=time; my @a=map $_,map $_,1..5e5;say
+ time-$t; <>"
7.87800002098084
[download]

8 seconds; 72MB; 1.5 million page faults.

Any thoughts where to start looking?

This is Perl 5.10.1 (AS1007) 64-bit on windows. Other platforms are almost certainly not affected.