#!/usr/bin/perl
use strict;
use feature qw{ say };
use warnings;

use Unicode::Normalize qw{ normalize };
use Unicode::GCString;

my $char = "\N{LATIN SMALL LETTER U WITH DIAERESIS}";
binmode *STDOUT, ':encoding(UTF-8)';
for (qw( D C )) {
    my $n = normalize($_, $char);
    my $gcs = 'Unicode::GCString'->new($n);
    say join ' ', length($n),
                  $n =~ s/(\X)/$1/g,
                  $1,
                  $gcs->chars,
                  $gcs->columns,
                  $gcs->length;
}
[download]

2 1 ü 2 1 1
1 1 ü 1 1 1

Update: Added the output.