Which version of Windows are you using and what code did you write?

I ask this because on Windows 10, the following works perfectly:

a.pl:

use 5.16.2;

system "start perl a.plx";
system "start perl a.plx";
system "start perl a.plx";

my $i = 1;
say $i++ while sleep 1;
[download]

a.plx:

use 5.16.2;

my $i = 1;
say $i++ while sleep 1;
[download]

The reason this works in Windows is because start spawns a new command prompt instance, completely detached from the parent instance. This separate instance can be terminated separately with ctrl-c, so if you're looking for something that is unkillable, you will have to look elsewhere (such as firing the code under a different user account, where the logged in user has no rights to terminate software running under the alternate account)