The unsaid base problem here is that someone started with the string "ต้มยำกุ้ง" but then incorrectly decoded it (probably to Windows cp1258) to create a new string of "ต้มยำกุ้ง"

Note that in UTF-8, each of the nine charcters in the Thai string takes three bytes. Therefore the Latin 1 decoding includes 9 x 3 = 27 characters and each triplet begins with �. It is often the case that an incorrectly decoded UTF-8 string into a Latin 1 character set will show each original character as beginning with some accented form of the letter a or A.

I was not able to repair the string in place, but writing it to a file, I can use Perl's IO Layers and the Encode module to repair the encoding.

use strict;
use Encode;
$|++;

my $t = 'thai.txt'; # contains => ต้มยำกุ้ง

open my $fh, '<:raw', $t
    or die "Couldn't open $t: $!";
my $content = do { local $/; <$fh> };
close $fh;

$content = decode('UTF-8', $content);

binmode *STDOUT, ':encoding(UTF-8)';
print "$content\n";
[download]

However, note that rather than writing this program you can use Perl's wonderful character encoder/decoder without writing any code:

    piconv -t UTF-8 thai.txt > thai_fixed.txt
[download]