Heres an example. You can also use select on the filehandles.

#!/usr/bin/perl
use warnings;
use strict;
use IPC::Open3;

my $cmd = 'ls -la';

my $pid = open3(\*WRITER, \*READER, \*ERROR, $cmd);
#if \*ERROR is 0, stderr goes to stdout


while( my $output = <READER> ) {
             print "output->$output";
      }

while( my $errout = <ERROR> ) {
             print "err->$errout";
      }

waitpid( $pid, 0 ) or die "$!\n";
my $retval =  $?;
print "retval-> $retval\n";
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