May I suggest

use CGI::Carp 'fatalsToBrowser'; # at the top of your script
...
open(my $RESULTS_FILE...) or die "Error opening results file: param $r
+eturned_param, data $data, error $!\n";
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Also, it would be nice to see what your parameters actually look like. What happens if you open just one file outside the loop and print "data $data, param $returned_param\n" to it?