I guess I don't understand modules

This bit of test code works as expected:

#!/usr/bin/perl
use Audio::TagLib;
use warnings;
use strict;
my $f      = Audio::TagLib::FileRef->new("_tune.mp3");
my $artist = $f->tag()->artist();
print $artist->toCString(), "\n";

but when I put that line into a subroutine in my program:

sub dofile {
        my $f = shift;

        my $ft;
        my $fn;

        $f = $cwd . '/' . $f unless ($f =~ /^\//);    # absolute path
         if ($f =~ /(\.[mo][po][3g])$/) {
                my $ext = $1;
                $ft = Audio::Taglib::FileRef->new($f);

                $fn = $ft->tag()->title();

                doxfer($f, $fn . $ext );
        }
}

I get this error:

Can't locate object method "new" via package "Audio::Taglib::FileRef" 
(perhaps you forgot to load "Audio::Taglib::FileRef"?) at ./sndxfer line 
213.

Of course I have "use Audio::TagLib;" at the beginning of my program.
What else do I need to know to use CPAN modules?

thx,
stevem