Operator precedence determines the order in which the operations are applied. For instance, in Perl, the expression:
3 + 3 * 4
isn't evaluated left to right, which calculates 3 + 3 equals 6, and 6 times 4 results in a value of 24; the precedence rules cause the multiplication to be applied first, for a final result of 15. The precedence rules are available in the perlop manpage and in most Perl books. However, I recommend you use parentheses to make your code more readable and to avoid bugs. They make the following expressions unambiguous; the first:
(3 + 3) * 4
evaluates to 24, and the second:
3 + (3 * 4)
evaluates to 15.