my $stdout        = IO::Select->new();
my $stdout_handle = IO::Handle->new();
$stdout->add($stdout_handle);
my $pid = open3( undef, $stdout_handle, undef, $command );
...error handling...

##</code><code>##

my $stdout        = IO::Select->new();
my $stdout_handle = IO::Handle->new();
my $pid = open3( undef, $stdout_handle, undef, $command );
...error handling...
$stdout->add($stdout_handle);

##</code><code>##

>perl -e "print qq{abc\n}; warn qq{def\n};" 2>&1 | perl -pe "s/^/$. /"
1 def
2 abc