Perl file test

What is the differance between the -f and -T file tests in Perl.
My documentation says:

-f Entry is a plain file
-T File is "text"

I had used -T to test whether it is a "plain text (ASCII) file",
but it keeps returning false when I know it is true. Perhaps I do not understand
what is meant by 'File is "text".

Can anyone enlighten me?

v/r
-Mike