Perl will ignore any line that begins with a pound sign. You do not need to end your comments with semicolons. You can also use comments to tell Perl to ignore parts of your program. You might want to do this to track down a bug, or to bypass a part of the program that isn't working correctly. Scalar variables always begin with a dollar sign.
Numeric variables can only contain numbers and maybe a decimal point.
Perl doesn't care whether a variable is numeric or text until it uses it. If you try to use a variable as numeric when it can't be converted to a number, Perl assumes it is zero. And you can use parentheses to partition off parts of the equations. Multiplication and division, for example, take precedence over addition and subtraction. Two plus five times three is seventeen, not twenty-one.