# untested
use Net::OpenSSH;

my $ssh = Net::OpenSSH->new(...);
my ($pty, $pid) = $ssh->open2pty('microcom /tmp/dev/mux1');

my $exp = Expect->init($pty);
$exp->send("AT+CSQ\r\n");
$exp->expect($timeout,
             [qr/^OK$/ => sub { ... }],
             [qr/^ERROR$/ => sub { ... }]);
$exp->soft_close();
waitpid($pid, 0);