-<DD>Run <TT>cmd</TT> with a read pipe connected to its stdout.
- When data is received through the pipe, <TT>handler</TT> is called
- with that data.
+<DD>Run <TT>cmd</TT> with a read pipe connected to its stdout and stderr.
+ When data is received through one of these pipes, <TT>h</TT> or <TT>errh</TT>
+ is called with that data. When the pipe is closed, the handler is called
+ with <TT>nil</TT> argument. The PID of the new process is returned, or
+ -1 on error.