I just wrote something like that today.

The code flow went something like this:

if we haven't seen all the matches yet
    create a link to the next 50
if we're not starting at the 0th match
    create a link to the previous 50
[download]

To do this, I passed a param named 'next', just as you describe:

my $url = "$ENV{SCRIPT_NAME}/?next=$next";

I used $next as a parameter to the MySQL LIMIT command. If it's not passed as a parameter, I set it to zero.