rje has asked for the wisdom of the Perl Monks concerning the following question:

Many moons ago, I saw a clever Perl programmer use 'which'... ON THE FIRST LINE OF HIS SCRIPT ... to find the perl executable.

In other words, instead of:

#!which perl

...except it wasn't exactly like that. It was sort of like that. But not exactly.
So I'm trying to figure out what that command was.
I figure you guys probably know it.
Any ideas? Thanks.