You forgot to declare the variable $main_detail. Since it’s a meta field you need to retrieve it first before you add it.
$main_detail = get_post_meta($post->ID, '_main_detail', true);
Also, I can see that some errors in your code.
echo '<label >';?><?php _e( 'short description:' );?></label>
Should be..
echo '<label >' . _e( "short description:" ) . '</label>';
And, you also need to add the key _main_detail in your $performer_meta array in order to save it.