If you open a pipe on the command '-', i.e., either '|-' or '-|'
  with 2-arguments (or 1-argument) form of open(), then there is
  an implicit fork done, and the return value of open is the pid
  of the child within the parent process, and 0 within the child
  process. (Use defined($pid) to determine whether the open was
  successful.) The filehandle behaves normally for the parent, but
  i/o to that filehandle is piped from/to the STDOUT/STDIN of the
  child process. In the child process the filehandle isn't opened--
  i/o happens from/to the new STDOUT or STDIN.