(split(/\./,$_,3))[1] =~ /^(\d+)([A-Za-z]+)(\d+)$/; 
 [$_, $3, $months{$2}, sprintf("%02d", $1) ]
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So, to repeat what I said there...

You are using $1, $2, $3 without testing the success of the match. This means that you might possibly be getting the previous round's data, resulting in duplicated output. What you should be doing instead is skipping over the erroneous entries, or perhaps dieing. Just to be different from the previous node, let's do the die thing:

 (split(/\./,$_,3))[1] =~ /^(\d+)([A-Za-z]+)(\d+)$/ or die "improperly
+ formed data: $_";
 [$_, $3, $months{$2}, sprintf("%02d", $1) ]
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