All variables have to either be % (normal) or & (binary). $ is an identifier and not a variable.
You can set a temporary variable with:
/var %newvarname = %oldvarname
/var %newvarname = $remove($6,$chr(44))
Or, you'd use /set for a permanent variable. If you already have your variable set up with /var, you can leave off the /var part, but I personally find it easier to quickly read with /var or /set present.