If I understand this correctly:

Then the shell is required to ignore the #! line if present, since the kernel already tried it and failed. However, this section of the other page you linked to hints that bash may do #! interpretation on systems that don't support #!, such as cygwin.

So, I was completely off the mark for saying it might be in libc, but correct in the original response that it isn't the shell doing #! interpretation, since it's natively supported by macOS and Linux.

FWIW, here is what I used to use as a template, when I was working in a shop with Solaris, HP-UX, and Linux, and I couldn't count on the system's version of perl.

#!/bin/sh

PATH=$HOME/perl/bin:$HOME/bin:$PATH
export PATH

exec perl -S -w -x $0 ${1+"$@"}

#!perl
#line 10

use 5.008;
use strict;
use warnings;
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