In case the files you have to process are quite large, I recommend to limit the number of updates of your progress indicator. Maybe 10-100 updates per prcessed file are sufficient? The following snipped will update about 100 times:

#in the loop
# update $percentage, e.g. 100 * $summed_size / $file_size; 
...
(printf STDERR "Progress: %3d%%\r", $percentage),$lastpercentage=int($
+percentage) 
     if int($percentage) != $lastpercentage;
...
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Last given output might not be '100%'. You might want to force an update at the last iteration (using e.g. eof).