indivauth's regex doesn't allow opposite ordering of given and surnames. Consider instead:

use strict;
use warnings;

undef $/;

my $alt = '(?:sur|given)name';
my $content = <DATA>;

$content =~ s#([^<]*)(<$alt>.*</$alt>)#$1."<authorfield>".&indivauth($
+2)."</authorfield>\n"#egi;

sub indivauth
{
    my ($inline) = @_;
    $inline =~s#(<$alt>(?:(?!$alt|</au>).)*?</$alt>)#<au>$1</au>#gsi;
    return "$inline";
}

print $content;

__DATA__
<surname>Coff</surname>, <givenname>D.,</givenname>, <surname>Coff</su
+rname>, <givenname>D.,</givenname>
<givenname>D.,</givenname>, <surname>Coff</surname>, <givenname>D.,</g
+ivenname>, <surname>Coff</surname>
[download]

Prints:

<authorfield><au><surname>Coff</surname></au>, <au><givenname>D.,</giv
+enname></au>, <au><surname>Coff</surname></au>, <au><givenname>D.,</g
+ivenname></au></authorfield>

<authorfield><au><givenname>D.,</givenname></au>, <au><surname>Coff</s
+urname></au>, <au><givenname>D.,</givenname></au>, <au><surname>Coff<
+/surname></au></authorfield>
[download]