use Path::Class qw/file dir/;
my $startdir = dir('/Y/Music/');
my $curdir = dir('/Y/Music/Schubert/Lieder/Terfel');
my $plsname = join('_', grep {length}
    $curdir->relative($startdir)->components).".pls";
print "$plsname\n";

__END__

Schubert_Lieder_Terfel.pls