$cmd = "quote site ftp.example.com";

    ...

    local($rin);
    print CMD $cmd,"\r\n";

    vec($rin,fileno(CMD),1) = 1;
    for (;;) {
       if (($Timeout==0) || select($rout=$rin, undef, undef, $Timeout)) {
          unless(sysread(CMD, $buf, 1024)) {
             $Error = "Unexpected EOF on command channel";
             return undef;
          }
       }

    ...

    }