use strict;
use IPC::Open3 ;
use Data::Dumper ;
my $auth ="admin1" ;
my $pwd = "admin1" ;

close(STDOUT) ;
my $cmd = "net rpc rights list -U $authname%$pwd" ;
my ( $wfh,$rfh,$efh,$pid,@privilege) ;
$pid = open3( $wfh ,$rfh, $efh, $cmd );

open(FH,">/tmp/debug") ;

waitpid( $pid, 0 ) ;
if ( $? == 0 ) {
    print FH "Cmd OK\n" ;
    while(<$rfh>) {
       print FH $_ ;
    }
}
close(FH) ;

##</code><code>##

open OUT, ">/tmp/out.tmp" or die "Can't open OUT" ;
$pid = open3( $wfh ,\*OUT, $efh, $cmd );
... ...
while (<OUT>) {
   print FH $_ ;
}