In addition to what tilly said about select, in this case there is an additional, and often useful technique to actually SEPARATE the stdout and stderr stream. In IPC::Open3, if you set the \*ERR handle to 0, it will go to stdout, like in IPC::Open2. But what if you wanted your stderr to go somewhere different, or be highlighted in different colors, or go to a log file?. Then the general purpose techique shown above can be used. It is almost the way you would do it in c. Most people forget about select, because many modules handle those details transparently for them.

But it is a cute code fragment, isn't it? You know, us old geeks, drool over elegant time-tested code fragments. :-)