From perldoc -f -X :

The "-T" and "-B" switches work as follows. The first block or so of the file is examined for odd characters such as strange control codes or characters with the high bit set. If too many strange characters (>30%) are found, it's a "-B" file, otherwise it's a "-T" file. Also, any file containing null in the first block is considered a binary file. If "-T" or "-B" is used on a filehandle, the current stdio buffer is examined rather than the first block. Both "-T" and "-B" return true on a null file, or a file at EOF when testing a filehandle. Because you have to read a file to do the "-T" test, on most occasions you want to use a "-f" against the file first, as in "next unless -f $file && -T $file".