It's not a matter of "trusting". The kernel will only do something with the shebang line if 1) it's a Unix kernel and 2) the program was executed directly. Furthermore, not every kernel will do the same - different kernels have different limits on the number of significant characters.

So, to make things equal, perl will pay attention the the first line - if it's a shebang line, it will look which command follows the shebang line. If it doesn't look like a perl, it will start this command, else it will look which options have been passed.

It's also not a matter of re-reading. It has to read the first line anyway. (Other interpreters would just discard the first line - it's not a coincidence that interpreters (*) line Perl, shells and Python use the # as their comment character.

(*) Called interpreter because they can read a program on their STDIN and execute it.