Similar to yours and choroba's solutions but without string multiplication ('x'): 
my $pchr = "KQRBNP";
my $w1 = "QRKPNBNBQRK";
my $w2 = "";

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

print "$w1 -> $w2\n"

[download]
And a bit similar to drclaw's idea: 
my $pchr = "KQRBNP";
my $w1 = "QRKPNBNBQRK";
my $w2 = "";

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

print "$w1 -> $w2\n"

[download]
Not sure if regex is optimized in some versions and this code fails.

In reply to Re^2: Sort list by position of items in another list by rsFalse
in thread Sort list by position of items in another list by gflohr

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