my $str = "<P_contrib-author>Mariel Miller</P_contrib-author>,<P_contrib-author>Allyson Fiona Hadwin</P_contrib-author>,<P_contrib-author>Jenna Cambria</P_contrib-author>";

my @rawauthours = split( ",", $str );

my @authours = ();
foreach (@rawauthours) {
    $_ =~ /<P_contrib-author>(.*)<\/P_contrib-author>/;
    push( @authours, $1 );
}

print '<alt-title alt-title-type="running-head-verso">',
  join( ", ", @authours ),
  '</alt-title>';