The case of the supersized shebang

Sort-of. It does a PATH seach internally. This may find some perl executable and it may even be the one supposed to execute a particular script. However, it might as well not. perl is a fairly stable execution environment, however, it has its share of "Let's break working stuff because it's just WRONG!" (that it works, presumably :->) people who add essentially random code changes[*] whose purpose doesn't seem to be known to anyone.

[*] Eg, starting with perl 5.16, xs functions can't be loaded via Dynaloader anymore unless a new keyword is added to the existing code. The documentation makes it very clear that loading xs-functions via Dynaloader Is Just Wrong[tm], but there's no positive justification for the change.

No, that is not what /usr/bin/env was invented for. /usr/bin/env was invented for setting up different environment variables - that's why it's called "env."