What is happening here is that you are passing in a reference to the subroutine 'cb' into the object $p which is a HTML::LinkExtor object.

By doing this $p can use the subroutine you provide so that you can customize the work being done on the extracted links. You seem to be printing them to a file, but lets say for example you wanted to build a spider, you could then write a sub spider { ... } that would traverse the links you find recursively. You would then create it with something like this:

 
use HTML::LinkExtor; 
my $p = HTML::LinkExtor->new(\&spider, "http://www.perl.com");

sub spider { 
    # code omited 
}

$p->parse_file("index.html");
[download]

Hope this helps.

-----------------------------------
Frank Wiles <frank@wiles.org>
http://frank.wiles.org