I cannot see anything obvious that might cause your output file to be empty -- other than perhaps there were no events that matched your criteria. Is that possible?

This is the error i am getting since the file app_output.txt is empty couldn't read C:\log_parsing\app_output.txt - at log_windows_new.pl line 444.

That still doesn't isn't the full error message.

Line 444 is:

     my @a = read_file($outputfile1)
     or die "couldn't read $outputfile1 - $!";
# ........................................^^
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The $! should mean that the full error message would be something like:

couldn't read C:\log_parsing\app_output.txt - No such file or director
+y at log_windows_new.pl line 444.
#.............................................^^^^^^^^^^^^^^^^^^^^^^^^
+^
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Or perhaps:code> couldn't read C:\log_parsing\app_output.txt - Permission denied at log_windows_new.pl line 444. #.............................................^^^^^^^^^^^^^^^^^ </code>

That piece of information would likely point to the source of the problem; but you haven't given us it?

I'd also suggest that you add $^E to line 44 (and other error messages). Ie:

     my @a = read_file($outputfile1)
     or die "couldn't read $outputfile1 - $! [$^E]";
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That may provide further clarification of the problem.