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my $pid = open2($rfh, $wfh, "ssh $opts $hostspec $cmd");

### Self-pipe init.
pipe(SIGRD, SIGWR);
# Make SIGRD non-blocking.
vec($rin, fileno SIGRD, 1) = 1;

my $byte = 1;
syswrite(SIGWR, $byte, 1);

my $nfound = select($rout = $rin, $wout = $win, undef, undef);
# Check $nfound for errors.
...
    # CHLD arrived, do waitpid(), modify some data, ...
  }
}