You might miss to ask for multivalued data. But I can not be sure, without the data.

    my @data = $cgi_query->param($returned_param);
    # ^^^
    $returned_param =~ s/\d//g;    #Strip digits from parameter so tha
+t filenames match
    open(my $RESULTS_FILE, ">>C:/Inetpub/wwwroot/cgi-bin/results/$retu
+rned_param.txt");
   my $data = join "\n", @data;
   # ^^^^^^^^^^^^^^^^^^^^^^^^
   print $RESULTS_FILE $data;
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