• however, in any event Perl will parse the #! line itself and honour switches and stuff. So, if the system doesn't do it, or you run your program by (for example) perl my_wonderful_program.pl ...., you still get the switches from the #!.

Yes but the -T switch wants to be in the command line too. perldiag explains that this is because when perl sees the -T switch, it's too late to taint some things.