use IPC::Open3 qw( open3 );

sub launch_in_background {
   my ($prog, @args) = @_;
   open(local *CHILD_STDIN, '<', '/dev/null') or die $!;

   if (!@args) {
      # Make sure $prog isn't interpreted as a shell command.
      @args = ('-c', 'exec "$1"', 'dummy', $prog);
      $prog = '/bin/sh';
   }

   return open3('<&CHILD_STDIN', '>&STDOUT', '>&STDERR', $prog,@args);
}

my $pid1 = launch_in_background('prog', 'file1');
my $pid2 = launch_in_background('prog', 'file2');