Current Perl documentation can be found at perldoc.perl.org.

Here is our local, out-dated (pre-5.6) version:

See Laundering and Detecting Tainted Data. Here's an example (which doesn't use any system calls, because the kill() is given no processes to signal):

    sub is_tainted {
        return ! eval { join('',@_), kill 0; 1; };
    }

This is not -w clean, however. There is no -w clean way to detect taintedness - take this as a hint that you should untaint all possibly-tainted data.