I bet the continuously available fileevents are taking higher priority than the idleevents which is where the actual repaint is done. As Tk::Widget says:

       $widget->idletasks
           One of two methods which are used to bring the application 
+``up to
           date'' by entering the event loop repeated until all pendin
+g events
           (including idle callbacks) have been processed.

           If the idletasks method is specified, then no new events or
+ errors
           are processed; only idle callbacks are invoked. This causes
+ opera-
           tions that are normally deferred, such as display updates a
+nd win-
           dow layout calculations, to be performed immediately.

           The idletasks command is useful in scripts where changes ha
+ve been
           made to the application's state and you want those changes 
+to
           appear on the display immediately, rather than waiting for 
+the
           script to complete. Most display updates are performed as i
+dle
           callbacks, so idletasks will cause them to run. However, th
+ere are
           some kinds of updates that only happen in response to event
+s, such
           as those triggered by window size changes; these updates wi
+ll not
           occur in idletasks.
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