$mw->Button(text => $tasks->{"task"}->[$_]->{"description"}, command =
+> sub {pub("$_")})->pack();
[download]

Or ...

$mw->Button(text => $tasks->{"task"}->[$_]->{"description"}, command =
+> [\&pub, "$_"])->pack();
[download]

I belive the problem is that Tk is passing you the value of $_ at the time the button is pressed, and it appears you want the value of $_ from the time the button is created.