my $out;
my $err;
my $pid = open3(*INFH, *OUTFH, *ERRFH, $cmd);

my $sel = new IO::Select;
$sel->add(*OUTFH, *ERRFH);

while(my @ready_to_read = $sel->can_read)
{
  foreach my $fh (@ready_to_read)
  {
     my $line;
     my $len = read($fh, $line, 4096);
     if(!defined($len))
     {
        die "Whoops: $!";
     }
     elsif($len != 0)
     {
        if(fileno($fh) == fileno(ERRFH))
        {
           $err .= $line;
        }
        elsif(fileno($fh) == fileno(OUTFH))
        {
           $out .= $line;
        }
     }
     elsif($len == 0)
     {
        $sel->remove($fh);
     }
  }
}

close(OUTFH);
close(ERRFH);
close(INFH);
waitpid($pid, 0);

# $err and $out are now ready for doing whatever you want
# the return code from the command is available via '0xffff & $?'