As Ashkrynt said in other words, the reason $md5 returns "kaboom" is that $md5 and $md5() are two different identifiers, as far as mirc is concerned. It appears that the ~ in $~blah bypasses the alias of the same name, but this wouldn't make any difference, as built-in identifiers have priority over custom ones (the opposite situation between built-in /commands and aliases). The closest thing to a usage of ~ would be script support for future identifiers. Eg,
//echo -a In a future mirc version $!~qwerty will be implemented and this will not be empty: $~qwerty
So in any version prior to that hypothetical future version, $~qwerty will always be $null, even if you have a custom identifier named qwerty. But considering that we never get announcements for future identifiers, I don't see who would use this.
