Are you sure there are no problems with opening the file? You should always check the return value of open:

open(DAT, $fileName) || die "Could not open $fileName - $!\n";
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Also bear in mind that you are slurping the entire file into an array which will be ok for small files but might present a memory problem if the files you are reading get larger. I would probably do something like this:

open(DAT, $fileName) || die "Could not open $fileName - $!\n";
while (<DAT>) {
   print $_, "\n";
}
close(DAT);
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-- vek --