use Lingua::EN::NameCase qw( nc );

use strict;

while (<DATA>) {
	s/(?<=AU: )(.*)/nc $1/e;
	print;
}

__DATA__
AU: jane smith
AU: JOHN DOE
AU: keith miller
AU: PHILLIP LEIGH-WILLIAMS
AU: PAUL MCCARTNEY
AU: DOUGIE O'CALLAGHAN
AU: MARIE ST. JOHN
AU: henry viii

##</code><code>##

AU: Jane Smith
AU: John Doe
AU: Keith Miller
AU: Phillip Leigh-Williams
AU: Paul McCartney
AU: Dougie O'Callaghan
AU: Marie St. John
AU: Henry VIII