No idea if the condition is actually correct. But most obvious fault is that you're not specifying it to only work when $nick is twitchnotify.
Well, that and your script is only ever triggered when the text is exactly "just subscribed!". Specify it to allow wildcards both before and after the sentence if you want it to work.
Also, assuming that twitchnotify is the nick that gives you this message, you would simply welcome that bot several times over since you're using $nick and not $3 or wherever the real user is placed in the message.
