With match global, you can just assign the authors to an array of @authors (no while loop needed).

There appear to be some special cases for formatting..0,1,2,3,more authors. Some simple "if" logic could solve that.

The more general situation would be names like: Bob Smith, MD; Porterhouse, A. B.; Freddie, Jr. or some such name with commas in it. Often ";" is used instead of commas so that it is possible to re-parse this concatenated line back into the original names. You should consider the ramifications (if any) of creating a string that is difficult for a program to re-parse.

Update: To the best of my knowledge, the use of ";" as a separator when one or more names contain commas, is proper English grammar. I would suggest scalar grep.  if (grep{/,/}@authors){ #use ';' instead of ',' for the separator.}

#!/usr/bin/perl -w
use strict;

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 @authors = 
   ($str =~ /<P_contrib-author>\s*(.*?)\s*<\/P_contrib-author>/g); 
   
if (@authors >= 3)
{
   print join (", ", @authors[0..@authors-2]), 
         " and $authors[@authors-1]\n";
}
elsif (@authors == 2)
{
   print "$authors[0] and $authors[1]\n";
}
else
{
   print "$authors[0]\n";
}
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