How to rescan for mode comment in new buffer?

[KARR, DAVID (ATTCINW)]

If I've created a new empty file and pasted in the header that will make
the shell and Emacs recognize it as a Perl script, what's the easiest
way to "refresh" it so that Emacs recognizes that it has to go into Perl
mode?  I could just manually run "perl-mode", but I'd rather a cleaner
way to do it.  Saving, killing the buffer and reloading it seems
excessive also.