For casual scripts, I've always used the following:

eval '(exit $?0)' && eval 'exec perl -w -S $0 ${1+"$@"}'
                  && eval 'exec perl -w -S $0 $argv:q'
                  if 0;

# The above invocation finds perl in the path, wherever it may be
[download]

(I've forgotten where I stole this from.)

Is there a "best" way to get whichever perl is in your path?

I hadn't seen the env version before, nice.

I also agree, in many cases the shebang line should point to a fixed Perl. I can even see an argument for not giving any shebang hints, and always invoking the perl binary explicitly from an external source.