use IO::Select;

$selector = IO::Select->new();
$selector->add(*CMD_ERR, *CMD_OUT);

while ($selector->count()) {
   my ($readable, undef, $error) = 
      IO::Select::select($selector, undef, $selector);

   $selector->remove($_)
      foreach @$error;

   foreach my $fh (@$readable) {
      ...
   }
)