So: is what you want to do to count the number of (non-epoch) lines in the earlier file that are before the first date in the second file?
in reply to Re^4: Time matching YYYY-MM-DD HH:MM:SS.SSS
in thread Time matching YYYY-MM-DD HH:MM:SS.SSS
Note that unless the first timestamp in the later file exactly matches one of the timestamps in the earlier file, your code will keep counting through the rest of the file.
Use the 3-arg form with lexical filehandles:
If use warnings was in scope where you reference @Array0, you would have seen a warning (not that it affects the correct execution of your code).
open my $fh, '<', 'newfile.txt'
or die "Can't open newfile.txt: $!\n";