I believe the main test here needs to be for "", not for definedness. DBI is going to give him an array, and some of the values in it may be empty strings, if I understand the problem correctly. The empty strings are what needs to be replaced. I think this variation will do the job:

my @results_tmp = map {($_ ne "") ? "$_" : "--"} @results;
print join(",", @results_tmp),"\n";
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For those who aren't comfortable with map, this is about the same as:

my @results_tmp = @results;
($_ eq "") && ($_ = "--") for @results_tmp;
print join(",", @results_tmp),"\n";
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