I'd quit messing with file names and just go by mtime. Here's a way to make an AoA of file names.

my @files;
while (<*.txt>) {
    push @{$files[ (localtime +(stat)[9])[4] ]}, $_;
}
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That gives you the November files as @{$files[10]}. You may want to rewrite that to take account of years, too. A hash may be more convenient than an array.