While your conclusion is correct, Microsoft is not at all to blame for this. The extended hashbang is there so you can run your Perl script with almost any /bin/sh under the sun, even a shell/kernel that does not honor the hashbang and it will still launch perl in the end.

There is a similar approach for Windows and cmd.exe that makes a Perl program appear to be a valid .cmd file as well, but that approach is distinct from the approach taken for unixish environments.