One technique that can help manage the time a file is open is to use lexical file handles:

...
    {
    open my $fileIn, '<', $filename or die "Failed to open $filename: 
+$!\n";
    ...
    }
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The end of the enclosing block (the '}') is the place where the lexical file handle ('$fileIn') goes out of scope and is closed if it hasn't been closed already.