use strict;
use warnings;

$_ = join '', <DATA>;
push @_, "$2 $1" while m|<name><surname>(.*?)</surname><given-names>(.*?)</given-names></name>|g;
print '<P_contrib-author>' . join(', ', @_) . '</P_contrib-author>';

__DATA__
<contrib-group>
<name><surname>Wigfield</surname><given-names>Allan</given-names></name>
<name><surname>Klauda</surname><given-names>Susan L.</given-names></name>
<name><surname>Cambria</surname><given-names>Jenna</given-names></name>
</contrib-group>