How do I get $len as 1 in this example?

Works for me:

$ cat lig.t
use strict;
use warnings;
use utf8;

use Test::More tests => 1;
my $char = "ffi";
my $len = length($char);
is ($len, 1);
$ perl lig.t
1..1
ok 1
$ perl -v

This is perl 5, version 20, subversion 3 (v5.20.3) built for x86_64-linux-thread-multi
(with 16 registered patches, see perl -V for more detail)

If you are working with these sorts of characters you could do a lot worse than go through the length() miscounting UTF8 characters? thread.