-T's job is to turn on taint mode. When on the shebang line, it does not do its job. It may be understandable that it doesn't work, but it cannot be placed on the shebang line instead of the command line as previously stated.

??? It dies, protecting your program from running without taint, ie it works.

I don't see that anywhere in the documentation. That's not it's job. It's like saying exit is the best debugging tool since it prevents all bugs!

No. What it is supposed to do is to run the script and die if the script attempts to do anything dangerous with not validated input. The "This is an error because, by the time Perl discovers a -T in a script, it's too late to properly taint everything from the environment. So Perl gives up." in perldiag seems to suggest that this is an implementation issue rather than a deliberate decision. Which is a shame. It would make a lot of things much easier.