When scripting, what's the difference between #!/usr/bin/perl and #!/usr/bin/env perl?

Obviously this applies equally with python, bash, sh, etc substituted for perl!

Quentin's answer below was clearly correct, and so I've accepted it, but I guess what I actually meant was 'what are the pros and cons of the two ways of using #! to invoke perl/python/bash as the interpreter of a script?'