Win8 Strawberry 5.8.9.5 (32)  Tue 06/27/2023  4:54:38
C:\@Work\Perl\monks
>perl

use strict;
use warnings;

use Data::Dump qw(dd);

my $seq = 'ddBddddBBBBBBBBBBBBDDDDDDDDBBBBBBBBBBBBBBddddddddddBBBBBBBB
+BBBBBBDDBBBBBBBBddddddddd';

my $rx_sub_str = qr{ B+ }xms;

my @endpoints;
while ($seq =~ / $rx_sub_str /xmsg) {
    push @endpoints, [ $-[0], $+[0] ];
    }

dd \@endpoints;

^Z
[[2, 3], [7, 19], [27, 41], [51, 65], [67, 75]]
[download]

Use of index might be faster than a regex approach for large data.