My first thought was, 'Silly guy, why is he writing C code? Doesn't he know the Perl(tm) way?'.

Then I thought, sprintf() is expensive .... how fast is it to run the integer math?

There is a subtle, but important difference between

    $var1 = sprintf "%.2f" => $number;
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and

    $var2 = int ($number * 100 + .5) / 100;
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$var1 is a PV, that is, the interval variable has a string value, but not a numeric value, while $var2 is an NV, that is, it has a numeric value, but not a string value. This means that $var1 will have two digits after the decimal point when printed - sprintf has garanteed that. But that's not necessarely true for $var2. Since you cannot represent 1/100 exactly in binary, you left the possibility open that if you stringify the number, you end up with more than 2 characters after the decimal point. Now, it may not happen in a thousand testcases, but can you guarantee it will never happen?

See perldoc -q decimal.