#!/usr/bin/perl

use strict;
use warnings;

my $string = 'BC BR BD POINT_A DN HN JN BR NK BR POINT_E BR NH BR CJ POINT_A MK NE BR BD BR POINT_E BR BD';

my @parts = split /(POINT_[AE])/,$string;

for( my $i = 2; $i < $#parts; $i+=4 ) {
    $parts[$i] =~ s/\bBR\b/YY/g;
}

$string = join '', @parts;
print $string;