I think unpack is better for this as haukex has suggested, but here's a pure-regex solution: 

Win8 Strawberry 5.8.9.5 (32)  Fri 01/15/2021 18:04:55
C:\@Work\Perl\monks
>perl -Mstrict -Mwarnings -MData::Dump=dd

my $s = q(AAAAbbbbCCCCddddXeeeeeeeeeeX);

my @caps = $s =~ m{ (?<! \A .{16}) \G .{4} | .* }xmsg;
dd \@caps;

^Z
["AAAA", "bbbb", "CCCC", "dddd", "XeeeeeeeeeeX", ""]

[download]
The trick is to have an unambiguous look-around anchor.

Update: Another variation: 

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", ""]

[download]

Give a man a fish:  <%-{-{-{-<

In reply to Re: mysteries of regex substring matching (updated) by AnomalousMonk
in thread mysteries of regex substring matching by smile4me

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