Unrelated to the problem which you already fixed... here are some tips (which you might already know)

You can save yourself some typing by eliminating those quotes and commas. Use qw as a list constructor.

my @unsorted = qw(
                864278_864377
                864518_864703
                851171_851270
                855398_855579
                856258_856357
                861015_861139
                866387_866549
                791806_792296
               1088806_1082296
);
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Also, the Core Data::Dumper module is handy for printing arbitrary data structures:

use Data::Dumper;
print Dumper(\@sorted);
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