Do read perlsub if you meant to make a function. There are math examples you may emulate within perlsub (just search for the string "cube_root" within perlsub to get a feel for what you're trying to accomplish).
FWIW,if you were not trying to create a function, and instead merely set a variable to calculate profit you would have the following (note: I changed the equation a lot):
my $net = 123.456;
my $coffee_allowance = 100.00;
my $profit = (
$net - $coffee_allowance
);
my $step_three = $profit;
print "profit = $step_three \n";
(minor code updates)