my $pid = open my $rdr, "$cmd |" or die "unable to run external command: $!";

while (<$rdr>) {
   ...
}
close $rdr or die "reading from $rdr failed: $!";


##</code><code>##

my $pid = open my $rdr, '-|', $cmd, @args or die "unable to run external command: $!";

while (<$rdr>) {
   ...
}
close $rdr or die "reading from $rdr failed: $!";