Yes you are right, there is a problem that my code does not pick out multiple authors. I have omitted multiple authors for being lazy.

Fixing the code is simple though, just modify the code slightly to read multiple authors (same as multiple keys).

my @author = $rec =~ m/<key>\s*([^<]*)(?=<)/g;
$col{author} = \@author;
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And how to store the returned hash structure by the subroutine needs to be revised too since there can be multiple authors. That should be a simple exercise.