There is also a more compact way to do this called the Schwartzian Transform. In this case that comes out to the script:

#! /usr/bin/perl
print
  map {join ",", @$_}
    sort {$a->[2] cmp $b->[2] or $a->[0] <=> $b->[0]}
      map {[split /,/, $_, 3]} <>;
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which should be read after reading about map, and should be read bottom up. You would call it like this:

perl scriptname < input_file > output_file
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