You have ...

if $flag then $refsize=`getsize()` # getsize is a subprogram   
                                   # that gets the size of the file
$newsize=$refsize;
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How does getsize() know which file to process?

When you call getsize(), a new shell is started up, configured, the getsize program is loaded from disk or from cache, and then it runs. That takes a relatively long time, and occurs often, once for each file. Why not use the built-in stat() function. You still have one disk access, either way, but save an unneccessarily wasted quarter second per file. Optimizing too early is a mistake, but there's no need to throw away time doing things that are already built into your language.