Found in C:\Perl\lib\pod\perlfaq8.pod
  How can I capture STDERR from an external command?
    There are three basic ways of running external commands:

        system $cmd;                # using system()
        $output = `$cmd`;           # using backticks (``)
        open (PIPE, "cmd |");       # using open()

    With system(), both STDOUT and STDERR will go the same place as the
    script's STDOUT and STDERR, unless the system() command redirects them.
    Backticks and open() read only the STDOUT of your command.

Here's another snippet:
-----------------------

    You can also use file-descriptor redirection to make STDERR a duplicate
    of STDOUT:

        $output = `$cmd 2>&1`;
        open (PIPE, "cmd 2>&1 |");

And another snippet:
--------------------

    ...To capture a command's STDERR and STDOUT together:

        $output = `cmd 2>&1`;           # either with backticks
        $pid = open(PH, "cmd 2>&1 |");  # or with an open pipe
        while (<PH>) { }                #    plus a read