The printf is used to ensure that should the line numbers jump from single digit to two

The sprintf pads the lines numbers with leading zeros so the sort correctly. Ie so that they do not sort as:

1
10
11
...
2
20
21
22
...
3
30
31
32
33
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%05u means we can correctly sort files with upto 99999 lines. If you need more change the number in the format.

Does the sort at the end assume that the input ( and the required output after removing rows ) is sorted

Why would sort "assume its input was sorted"? We are sorting them because we know they will not be. That's why we added the line numbers so that we can put the, back into the input ordering. Perhaps the following where I've left the line numbers in place will clarify things?

C:\test>perl -anle"$h{ $F[1] } = sprintf qq[%05u%s], $., $_; }{ print 
+for sort values %h"
1/2/2013     cgoo     nreuiheru
1/4/2013     doow    reiqrqueih
1/5/2013     hellio    ruieqrhfuepqh
1/20/2013    cgoo    3rhquh4ureyh
1/30/2013    yetil     jerqohgqrij
2/13/2013    hellio   rueqipheruh
2/14/2013    cgoo    wehrig4r74378
^Z
000021/4/2013     doow    reiqrqueih
000051/30/2013    yetil     jerqohgqrij
000062/13/2013    hellio   rueqipheruh
000072/14/2013    cgoo    wehrig4r74378
[download]