You can get away from receiving an error without brackets or parentheses in three ways, but you can't lose both of them:
Code:
if ($nick == $chr(95)) ;My cmd
Or
Code:
if $nick == $chr(95) { ;My cmd }
And Of course, the traditional way with both intact:
Code:
if ($nick == $chr(95)) { ;My cmd }
It's always a good practice to include the brackets and parentheses whenever possible.