my $pchr = "KQRBNP";
my $w1 = "QRKPNBNBQRK";
my $w2 = "";

for my $L ( split //, $pchr ){
  $w1 =~ m/$L(?{ $w2 .= $L })(*FAIL)/;
}

print "$w1 -> $w2\n"

##</code><code>##

my $pchr = "KQRBNP";
my $w1 = "QRKPNBNBQRK";
my $w2 = "";

my $re = join "|", map { ".*$_" } split //, $pchr;
$w1 =~ m/^(?:$re)(?<=(.))(?{ $w2 .= $1 })(*FAIL)/;

print "$w1 -> $w2\n"