I'd start with a dummy child, make the other children wait for it, and then kill it e.g.

#!C:\Perl\bin\perl.exe

use warnings;
use strict;

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

my $pid0 = open my $nh, 'type nul |'; # spawn dummy child
my @pid;
for ( @drive_dir ) {
   my $pid;
   $pid = fork
     and push @pid, $pid;
   unless( $pid ) { # non-dummy child
      waitpid $pid0, 0; # wait for dummy child to die
      system( 'perl $_' );
      exit;
   }
}
# parent
sleep 1; #allow children a little time to start
<$nh>;
close $nh; # kills dummy child
waitpid $_, 0 for ( @pid ); # keep parent alive until all children don
+e
[download]