One way to begin attacking a problem is to just start writing down obvious (update: even rather trivial) elements of the problem and general relationships between those elements. Even if it doesn't seem immediately relevant, just write it down. As you begin to see more relationships, write those down (or at least keep them in mind). AnonyMonk has started the process here.

What else springs to mind?

• If each pizza has a cost and you make a certain number of them, what's the total cost?
      my $total_cost = $cost ... $sold ...;
  
• If you're actually selling each pizza at a discounted price, what is that price?
      my $discounted_price = $price ... $discount ...;
  
• If you sell a certain number of pizzas at the discounted price, what are your total sales?
      my $total_sales = $discounted_price ... $sold ...;
  
• What's your profit?
      my $profit = $total_sales ... $total_cost ...;
  

So you might end up with an outline of general relationships like this:

#!/usr/bin/perl

use strict;
use warnings;

my $sold  = 10;
my $cost  = 250;
my $price = 300;

my $discount = 0.10;

my $total_cost = $cost ... $sold ...;

my $discounted_price = $price ... $discount ...;

my $total_sales = $discounted_price ... $sold ...;

my $profit = $total_sales ... $total_cost ...;
[download]

Again, as long as they're true, don't worry if some of the statements don't seem immediately relevant or useful; it's easy to delete text. Work on refining relationships that seem useful: exactly what is the relationship of the total cost to the per-item cost and the total made or sold? Continue on to a solution!