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my %order;
@order{qw(Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec)} = (1..12);
...
my @sorted_filenames = sort {
  $order{substr($a,0,3)} <=> $order{substr($b,0,3)}
} @unsorted_filenames;

my %piles;
for (@unsorted_filenames) {
...
  delete $piles{$_};
}
die if keys %piles; # unexpected prefix!