Actually, perl does always parse the #! line, for two reasons:

• To work around old, buggy, or limited kernels that pass the arguments from the #! line incompletely themselves, or not at all.
• As a security precaution: if a script says it wants taint checks in the #! line (by having -T included there), but the -T switch was not in fact found on the real command line, perl will complain and die rather than run without taint checks.

The relevant quote from the perlrun manpage:

The #! line is always examined for switches as the line is being parsed. Thus, if you’re on a machine that allows only one argument with the #! line, or worse, doesn’t even recognize the #! line, you still can get consistent switch behavior regardless of how Perl was invoked, even if -x was used to find the beginning of the program.

(but it's certainly an ugly hack that it does this. After all, the idea of the shebang was that the interpreters so invoked wouldn't even be aware of its existence.)