You have at least one logic error in your program. You will only ever fork once and then exec the first task. The second and third task will never get started. See Parallel::ForkManager or runN for how to conveniently launch multiple programs from fork().

On plain Windows Perl, system(1, ...) starts a detached process. I don't know whether cygwin Perl also supports that.

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.

Something like this should work (this is an abbreviated version of my service control class).

Warning, this might/will need some fiddling from you. I pulled this out of my (DarkPAN) repo and removed much of the code to make the (for your problem) relevant parts a bit easier to understand.

First, you need to use some Win32 modules:

use Win32::Process;
use Win32;
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sub start_app {
    my ($self, $app) = @_;

    my $ProcessObj;
    if(!Win32::Process::Create($ProcessObj,
                               $app->{app},
                               $app->{app}." ".$app->{cmdlineopts},
                                0,
                                NORMAL_PRIORITY_CLASS,
                                $self->{basePath})) {
        print "Error starting app " . $app->{description} . ": " .
                Win32::FormatMessage( Win32::GetLastError() ) .
                "\n";
        $app->{handle} = undef;
        return 0;
    } else {
        $app->{handle} = $ProcessObj;
        my $app_pid = $app->{handle}->GetProcessID();
        print "Started app " . $app->{description} . " with PID " . $a
+pp_pid . "\n";
        return 1;
   }
}
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sub check_app {
    my ($self, $app) = @_;

    # Ignore if this application was never started
    # or was intentionally stopped
    if(!defined($app->{handle})) {
        return $self->start_app($app);
    }

    # First, check if the process exited
    if($app->{handle}->Wait(1)) {
        # Process exited, so, restart
        print "Process exit detected: " . $app->{description} . "!n";
        return $self->start_app($app);
    }

    # All ok, just return true
    return 1;
}
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sub stop_app {
    my ($self, $app) = @_;

    if(defined($app->{handle}) && $app->{handle}) {
        print "Killing app " . $app->{description} . "...\n";
        my $app_pid = $app->{handle}->GetProcessID();
        $app->{handle}->Kill(0);
        $app->{handle}->Wait(2000);
        $app->{handle} = undef;
        print "...killed.\n";
    } else {
        print "App " . $app->{description} . " already killed\n";
    }

}
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You don't need "fork", all you need is "spawn". Just because Unix spells "spawn" as "fork then exec" doesn't mean that is how you need to do it. Unfortunately, Perl doesn't expose a portable implementation of "spawn" despite having implemented one internally.

But if you have Windows Perl ("native" not cygwin), then your problem is solved as easily as:

    system( 1, $^X, $_ )
        for qw<
            /a/dir_fil­e_create.t­xt
            /b/dir_fil­e_create.t­xt
            /c/dir_fil­e_create.t­xt
        >;
    while(  0 <= wait()  ) {
        warn "Child exited with ", $?>>8, $/;
    }
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#!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
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