You might be able to find the answer in this tpj article (would have linked to the official archive but it always seems to hang in my browser)

I have to admit that I don't understand it enough to actually answer your question, but the article above ends with:

And now you know why perl 5.004 has the new warning Too late for -T option. Taint mode isn't effective unless it's done from the very start, i.e. from the real command line. By the time yylex is called, a lot of taint-related things should already have happened, so perl can't guarantee that your data is safe. To avoid this problem, always make -T the first option on the shebang line.

So, I assume the answer can be found somewhere in there....
time for me to go read it myself, now.

-Blake