what I see is nonsense to me

This is perl 5, version 26, subversion 0 (v5.26.0) built for MSWin32-x64-multi-thread

With your program (little modification to not watch the task manager) I see it doubled

use strict;
use Devel::Size qw(total_size);

my $x = 'a' x (2**30); 

print                   "Devel::Size        = ".human(total_size($x)).
+"\n";
open my $cmd, qq(tasklist /NH /FI "PID eq $$"|) or die;
while (<$cmd>){ print qq(tasklist PID $$ = $1\n) if /(\S+\s\w{1,2}$)/}

sub human{
    my $size = shift;
    my  @order= qw/Tb Gb Mb Kb byte/;
    if($size<1024){return"$size byte"}
    while  ($size >= 1024){$size=$size/1024;pop @order;}
    return sprintf("%4.2f %2s", $size, (pop @order));
}
__END__

Devel::Size        = 1.00 Gb
tasklist PID 37288 = 2.104.612 K
[download]

But with this version of mine I see what everyone is expecting to:

use strict;
use warnings;
use Devel::Size qw(total_size);

my $x; 

foreach my $order ( qw(20 24 30 32) ){
    $x = 'a' x ( 2 ** $order );
    print "\n\nsize of scalar 2**$order\n";
    print                   "Devel::Size        = ".human(total_size($
+x))."\n";
    open my $cmd, qq(tasklist /NH /FI "PID eq $$"|) or die;
    while (<$cmd>){ print qq(tasklist PID $$ = $1\n) if /(\S+\s\w{1,2}
+$)/}
}

sub human{
    my $size = shift;
    my  @order= qw/Tb Gb Mb Kb byte/;
    if($size<1024){return"$size byte"}
    while  ($size >= 1024){$size=$size/1024;pop @order;}
    return sprintf("%4.2f %2s", $size, (pop @order));
}

__END__

size of scalar 2**20
Devel::Size        = 1.00 Mb
tasklist PID 19660 = 8.452 K


size of scalar 2**24
Devel::Size        = 16.00 Mb
tasklist PID 19660 = 23.820 K


size of scalar 2**30
Devel::Size        = 1.00 Gb
tasklist PID 19660 = 1.056.012 K


size of scalar 2**32
Devel::Size        = 4.00 Gb
tasklist PID 19660 = 4.201.748 K
[download]

L*