4. Variables

Variables are named cells stored in the computer memory that can hold any single Perl value. One can change the value that a variable holds, and one can later retrieve the last value assigned as many times as wanted.

Variables in Perl start with a dollar sign ($) and proceed with any number of letters, digits and underscores (_) as long as the first letter after the dollar is a letter or underscore.

To retrieve the value of a variable one simply places the variable name (again including the dollar sign) inside an expression.

To assign value to a variable, one places the full variable name (including the dollar sign) in front of an equal sign (=) and places the value to the right of the equal sign. This form is considered a statement and should be followed by a semicolon. The value assigned may be an expression that may contain other variables (including the assigned variable itself!).

An example will illustrate it:

$myvar = 17;
$a = 2;
print $myvar, " * ", $a, " = " , ($myvar*$a), "\n";
$a = 10;
print $myvar, " * ", $a, " = " , ($myvar*$a), "\n";
$a = 75;
print $myvar, " * ", $a, " = " , ($myvar*$a), "\n";
$a = 24;
print $myvar, " * ", $a, " = " , ($myvar*$a), "\n";

The output of this program is:

17 * 2 = 34
17 * 10 = 170
17 * 75 = 1275
17 * 24 = 408

Several things can be noticed:

1. The value of $a changes throughout the program. It's perfectly fine, and usually even necessary to modify the value of a variable.
2. By using $myvar we can ensure, that assuming we wish to change its value, we will only have to change it in one place, not in every place it appears.