If you redefine your problem to be "I want to run N commands concurrently", then your entire program could be just:

use strict;
use threads;

my @drive_dir = (
    'perl /a/dir_file_create.txt' , 
    'perl /b/dir_file_create.txt', 
    'perl /c/dir_file_create.txt'
);

my @kids = map async{ system $_; }, @drive_dir;
$_->join for @kids;
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Which will work without requiring fork emulation or cygwin.