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parv
<p>For details, see [doc://open] and [doc://fork] functions to start.
</p>
<p>That <c>open</c> syntax forks the process (which I had long forgotten); <c>$mailprog</c> is run in child process. A pipe is opened to <c>$mailprog</c> in order to write to its standard input by writing to <c>$mailhandle</c>. (If the pipe-open syntax were <c>-|</c>, then that would be to read standard output of the command.)
</p>
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