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my $output_pipe = "|sort -n -t, -k3,4 | uniq > $tmpfile";
open(OUTFILE,$output_pipe) || die "could not open file $output_pipe fo
+r output: $!";

my $output_pipe = "|sort -n -t, -k3,4 -u > $tmpfile";

$ cat test.dat
1,1
...
2,2
2,1
1,1

$ sort -n -t, -k1,2 test.dat | uniq
1,1
...
2,4

That's good.

$ sort -n -t, -k1,2 -u test.dat

...
2,1

Not what I expected.