This is in perlfaq4, specifically How do I sort an array by (anything)?

In this case, you should probably look into making a more advanced data structure with split or using substr in a more complex sort function. Something like this may work:

my @sorted = sort { substr($a, 0, 4) <=> substr($b, 0, 4) } @unsorted;
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This code is untested, suitable only as an example of technique, not syntax.