Well, all perl builtins work at the codepoint level, including length. Depending on your definition of "character", that might or might not be what the OP wants.

Sure, I'm just saying that bugs or unexpected results can occur if care is not taken. As amon pointed out, the same visual representation of a character with a diacritical might have either one or two codepoints.

#!/usr/bin/env perl
use v5.14;
use warnings;
use utf8;
binmode STDOUT, 'utf8';

my $o_umlaut1 = "\x{F6}";
my $o_umlaut2 = "\x{6F}\x{308}";
my $string1 = "æð" . $o_umlaut1;
my $string2 = "æð" . $o_umlaut2;
say "length of $string1 is ", length($string1);
say "length of $string2 is ", length($string2);
[download]

__OUTPUT__
length of æðö is 3
length of æðö is 4

I'll play around with your module. Thai is somewhat unique in that the first combining character may be another alphabetic character, so counting extended graphemes does not necessarily give the correct count of alphabetic characters.