%letters = ('dny' => '#225',
            'kh'  => '#35',
            'k'   => '#12',
            'h'   => '#10',
            'a'   => '#8',
            't'   => '#7',
            'o'   => '#6',
            's'   => '#5');

$string = 'khatos';

#if you ever want to match more than 3 characters
#then change the 3 below to whatever number
#you want.
while($string =~ s/([a-zA-Z]{1,3})/&roman($1)/e)
 {}
print "$string\n";

sub roman
 {
   my $letter = shift;
   my $remainder;

   while(length($letter) > 0)
    {
      if(exists($letters{$letter}))
       {
         return $letters{$letter}.$remainder;
       }
      $remainder = chop($letter).$remainder;
    }
   #if the character isn't found in %letters it
   #will return '#?', if you return an alphabetic
   #character which isn't in %letters then the
   #program will loop forever.
   return '#?';
 }