osunderdog has already explained what the code does. The reason you are getting the warning is that the code starts to fill the array at index 1:

 for(my $bn=1;$bn<=$row[0];$bn++)
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but starts to retrieve and sort at index 0

@indexUlt=sort{$nxxult[$a]<=>$nxxult[$b]} 0 ... $#nxxult;
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Thus, you have an undef value in your comparison at nxxult[0] and you get a warning. Either change the loop to start filling the array at $nxxult[0] or count from 1 ... $#nxxult.