(\d\d\d\d_\d\d_\d\d_\d\d_\d\d_\d\d_\d\d\d)

Perhaps it's just me but I find all those \ds confusing and would prefer to use quantifiers.

(\d{4}(?:_\d\d){5}_\d{3})
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Also, the requirement was for a descending sort so I think

sort { $a->[1] cmp $b->[1] }
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sort { $b->[1] cmp $a->[1] }
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Cheers,

JohnGG

Update: I should have read the OP more carefully, the sort required is actually ascending as fenLisesi points out.