Another approach, if you really want to use a regex, would be to capture just that part that you're interested in:

my $somestring = "y:/home/lib/directory/piano_book";
($somestring) = $somestring =~ m#([^/]+)$#;
print $somestring;

__END__
piano_book
[download]

Update: or just to show that always TIMTOWTDI:

my $somestring = "y:/home/lib/directory/piano_book";
my $idx = rindex($somestring, "/");
$somestring = substr($somestring, $idx+1);
print $somestring;
[download]