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 for ($a=0; $a<=$#columns; $a++) {
      for ($b=0; $b<=$#columns; $b++) {
...
      }
      print " \n";
   }

my @AoA = ([1, 2, 3], [4, 5, 6], [7, 8, 9]); # an array of arrays for 
+testing purpose
for my $row (@AoA) {
...
    }
    print "\n";
}

1 2 3
4 5 6
7 8 9

my @AoA = ([1, 2, 3], [4, 5, 6], [7, 8, 9]);
for my $row (@AoA) {
...
    }
    print "Average: ", $sum / $count, "\n" if $count;
}

Average: 2
Average: 5
Average: 8