Works for me, Perl 5.8.2 on Linux. I wrote:

#!/usr/bin/perl
use utf8;

open my $fh,'<:encoding(sjis)','j.sjis.txt';
my $string='言葉';
print index(<$fh>,$string);

saving it as 'j.pl' as utf-8, and

日本語の言葉

saving it as 'j.sjis.txt' as shift-jis.

Running it:

$ perl j.pl j.sjis.txt
4

(yes, I used <pre> instead of <code>... but <code> would not let me use character entities, and without the proper characters this answer would be useless)

-- 
        dakkar - Mobilis in mobile