I would have done it differently. Personal preference dictates that I code like so:

my $str = "<P_contrib-author>Mariel Miller</P_contrib-author>,<P_contr
+ib-author>Allyson Fiona Hadwin</P_contrib-author>,<P_contrib-author>J
+enna 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>';
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If you want to make your script work, you should probably remove the willdcard(?), making it:

while($line =~ /<P_contrib-author>(.*)<\/P_contrib-author>/)
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...but I am not sure about that.