Send it to Perl how? Through CGI? Over a socket? From a file? To the script's STDIN? Those all require a slightly different approach, but generally you'll eventually end up with the whole JSON data in a variable, let's say $json. Then you use one of the JSON decoding mechanisms - I'd go for the first one in the synopsis that seems to match the description, so you'd have a line like $perl_data_structure = decode_json $json;.
Now, what $perl_data_structure looks like depends on what the outermost element of the JSON data was. Was it a map? Then $perl_data_structure will be a hash ref. Was it an array? Then $perl_data_structure will be an array ref. I am assuming you know what sort of data you want to send to your script and what it's formatted like, so this shouldn't be too much of an obstacle.
Inserting it into a database, again, can't happen without asking yourself some questions, first. What do the tables in the database look like? What tables are there, what columns do they have and what data types go into those columns? Which elements from the incoming data do I want to insert? How do I transform the data structure into SQL -- but note that at this point you're no longer dealing with JSON. All you have is just an ordinary complex Perl data structure.