Instead of using spawn, you have to send the command you want to run to the remote shell via its stdin channel: 
$expect->send("$command\n");

[download]

A better approach is to tell su to run the command directly using the -c flag: 

my ($pty, $pid) = $ssh->open2pty({stderr_to_stdout => 1}, 'su', '-c', 
+$command)
    or die "Can't open PTY: $!\n";

[download]

In reply to Re: Using Net::OpenSSH to su commands by salva
in thread Using Net::OpenSSH to su commands by aztlan667

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