I've tried to swap \n for \r\n

Like people have said, this should have worked. Notepad expects \r\n for line ends. Either 1) your file didn't use \n for line ends originally, or 2) you convertion code has a bug (and you probably ended up with \r\r\n).

A simple one liner to convert from \n to \r\n (when run from a Windows machine) is:

perl -pe1 infile > outfile
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The above is similar to the following code:

open(my $fh_in, '<', $file_in)
   or die("Unable to open $file_in: $!\n");

open(my $fh_out, '>', $file_out)
   or die("Unable to create $file_out: $!\n");

print $fh_out $_
   while <$fh_in>;
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