Win8 Strawberry 5.8.9.5 (32)  Fri 01/15/2021 18:19:20
C:\@Work\Perl\monks
>perl -Mstrict -Mwarnings -MData::Dump=dd

my $s = q(AAAAbbbbCCCCddddXeeeeeeeeeeX);

my $n = 4;
my $m = 3;
my @caps = $s =~ m{ (?<! \A (?: .{$n}){$m}) \G .{$n} | .* }xmsg;
dd \@caps;

^Z
["AAAA", "bbbb", "CCCC", "ddddXeeeeeeeeeeX", ""]