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";
[download]

(minor code updates)