Trimbach is politely trying to point out that you are missing both a common idiom and a feature of CGI.pm, namely that a query string without ampersands is treated as an ISINDEX search and the values can be retrieved with the keywords() method, though from my reading it's a list you'll need to join.

A small change to your code would make it work. I expect something more elegant could be done with a [0], but not by me :(

$page->param('field',join('',$page->keywords))
    unless $page->param('field');
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