Look:

$calc(9999999999999999) => 9999999999999998
$calc(3^40) => 12157665459056929000: False, 3^40 is equal to 1215766545905692801.

Instead of returning a false result, it would be more judicious to return nothing ($null) or an error message.

0 is just as incorrect as 12157665459056929000. By the way, $calc(2^1050) already implements your feature. Computer calculations always have a certain limitation in range and precision, if you really need better than what mIRc gives, google for big numbers or arbitrary precision math or something.
Maybe you can even write a mIRC dll using http://spanky.triumf.ca/www/fractint/arb-prec.html or something?

It has already been reported and explained here a month or two back.

Try using the Search link at the top of the page before posting.

Yes, here and here to, but I don't think that Khaled look the out-of-date messages before the release of a new version.

And my post isn't the same; read the last phrase of the post.