sub run_cmd
{
  my $cmd = shift();

  my $in;
  my $out;
  my $err;

  my $pid = open3($in, $out, $err, $cmd);

  return ($in, $out, $err);
}

my ($in, $out, $err)  = run_cmd("some command");

while(<$out>)
{
   do stuff with $out;
}

while(<$err>)
{
   do stuff with $err;
}