my %piles;
for (@unsorted_filenames) {
  push @{$piles{substr($_, 0, 3)}), $_;
}
my @result;
for (qw(Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec)) {
  push @result, sort @{$piles{$_}};
  delete $piles{$_};
}
die if keys %piles; # unexpected prefix!