because the map generates an anon array that is an "extra step" and that array is copied to @a.

No array is created. One million scalars are created, but the question wasn't about that snippet. It was provided as a baseline.

In your later code, you have a map within a map which is similar to a foreach within a foreach. So it is going to run like 1 million times slower.

No, it's not multiplicative like a foreach in a foreach. It's additive like a foreach after a foreach.

my @array = map A, map B, LIST;
[download]

is functionally similar to

my @list1;
for (LIST) {
    push @list1, B;
}
my @list2;
for (@list1) {
    push @list2, A;
}
my @array = @list2;
[download]

Snippet three should take about 0.21 + (0.21-0.06) = 0.56, but it's taking 31.48 due to excessive paging. Why is it paging so much?

on my machine for this, I get:

C:\Projects>perl -MTime::HiRes=time -wE"my $t=time; my @a=map{$_}map{$
+_}1..1e6;say time-$t;"
12.703125
[download]

which I think is similar to: my @array = map A, map B, LIST; I am actually surprised that on a large 64 bit machine running some kind of *nix, that there are any page faults at all. I mean why does the "simple" version page fault? Sorry, I don't know.

Update: oh I see that this is Windows, I presume Win 7 instead of Vista? There are a bunch of versions of this OS, that might matter.

more tests on my 32 bit Win XP Pro machine 2 GB memory, AS 5.10.1:

C:\Projects>perl -MTime::HiRes=time -wE"my $t=time; my @a=map{$_}map{ 
+$_}1..100e3;say time-$t;"
0.193822860717773

C:\Projects>perl -MTime::HiRes=time -wE"my $t=time; my @a=map{$_}map{ 
+$_}1..200e3;say time-$t;"
0.609375

C:\Projects>perl -MTime::HiRes=time -wE"my $t=time; my @a=map{$_}map{ 
+$_}1..300e3;say time-$t;"
1.28125

C:\Projects>perl -MTime::HiRes=time -wE"my $t=time; my @a=map{$_}map{ 
+$_}1..100e3;say time-$t;"
0.18930196762085

C:\Projects>perl -MTime::HiRes=time -wE"my $t=time; my @a=map{$_}map{ 
+$_}1..250e3;say time-$t;"
0.921875

C:\Projects>perl -MTime::HiRes=time -wE"my $t=time; my @a=map{$_}map{ 
+$_}1..500e3;say time-$t;"
3.375

C:\Projects>perl -MTime::HiRes=time -wE"my $t=time; my @a=map{$_}map{ 
+$_}1..1000e3;say time-$t;"
13.15625

C:\Projects>perl -MTime::HiRes=time -wE"my $t=time; my @a=map{$_}map{ 
+$_}1..2000e3;say time-$t;"
50.140625
[download]


Another "benchmark update":

C:\Projects>perl -MTime::HiRes=time -wE"my $t=time; my @a=1..16000e3;s
+ay time-$t;"
1.5

C:\Projects>perl -MTime::HiRes=time -wE"my $t=time; my @a=map{$_}1..16
+000e3;say time-$t;"
5.546875

C:\Projects>perl -MTime::HiRes=time -wE"my $t=time; my @a=map{$_}map{$
+_}1..16000e3;say time-$t;"
3173.625
[download]

The maps above essentially don't do anything useful at all, but this shows the exponential increase in execution time on my 32 bit Win machine. So, I don't think this is specific to 64 bit machines. Apparently map uses more memory than one would think for a "do nothing" operation and also that Perl winds up accessing this extra memory in a way that causes a lot of page faults which would indicate that Perl is not cycling through sequential memory locations. Why that is and how that works, I don't know yet. But at least I can say this happens on 32 bit machines also.

I am actually surprised that on a large 64 bit machine running some kind of *nix, that there are any page faults at all.

Are we sure there's actually even a problem? A page fault is not the same as paging (to swap). You'll take page faults any time you use memory, the first time you touch a page after you allocate it (on most *nix systems anyway; terms could mean totally different things on Windows, but I don't think they do).