@cmd = ('ssh', @ssh_options, # e.g., -o => 'ForwardX11 no'
        $user . '@' . $node, # e.g., root@serverC
        quote_cmd(@cmd) # this @cmd is the original command we want to run.
       );

@cmd = ('ssh', @ssh_options, # e.g., -o => 'ForwardX11 no'
        $ENV{PROXY_PORT} ? (-p => $ENV{PROXY_PORT}) : (),
        $ENV{SSH_HOSTNAME} ? ? (-o => "HostKeyAlias $ENV{SSH_HOSTNAME}") : (),
        $proxy, # localhost if A is needed, otherwise server B
        quote_cmd(@cmd)
       ) if $proxy;