C:\>perldoc -q stderr
Found in C:\Perl\lib\pods\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.

    You can also use the "open3()" function from "IPC::Open3". Benjamin
    Goldberg provides some sample code:

    To capture a program's STDOUT, but discard its STDERR:

            use IPC::Open3;
            use File::Spec;
            use Symbol qw(gensym);
            open(NULL, ">", File::Spec->devnull);
            my $pid = open3(gensym, \*PH, ">&NULL", "cmd");
            while( <PH> ) { }
            waitpid($pid, 0);

    To capture a program's STDERR, but discard its STDOUT:

            use IPC::Open3;
            use File::Spec;
            use Symbol qw(gensym);
            open(NULL, ">", File::Spec->devnull);
            my $pid = open3(gensym, ">&NULL", \*PH, "cmd");
            while( <PH> ) { }