One thing I see is that you are calling &modified_file with the new filename before you have renamed the file to that name.

# You don't have a file named $concat yet.
&modified_file($concat);
rename($orig, $concat) || print "error can't rename $orig to $concat: 
+$!";

#try switching the order:
rename($orig, $concat) || print "error can't rename $orig to $concat: 
+$!";
&modified_file($concat);
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That code snippet seems to work though. Are your print statements within that if block printing out? Or do you not even get to that point?