... 
my $j=Win32::Job->new;
$j->spawn('perl.exe', "perl.exe myprog.pl", {
       new_console=>0,
       new_group=>1,
       stderr=> $out,
       stdout=> $out
  })
unlink 'killfile.txt';
$j->watch(sub { -e 'killfile.txt' ? 1 : 0 }, 3);
print "go on\n";
...