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Variables
Like all languages, Perl uses variables to store values. In Perl all variables, without exception, begin with one of three characters:

• $ - defines a scalar (string, integer, floating point number)
• @ - defines an array
• % - defines a hash

The meanings of scalar, array and hash are generally well known, and will be discussed in more detail in the tutorial on data types.

Variable Names
Variable names (the part behind the $, @, and %) must consist of letters (a-z), numbers (0-9), or the underscore character. These are all valid variable names:

     $a
     $47
     @dog2
     @1down
     %my_hash
     %MyHash

Special Characters
Just so you won't be too confused when you see it, Perl also allows other special characters (such as ^ or &) to be used in single character variable names. Perl reserves most of these for its use so you won't be creating such variable names yourself.

Two such single character, pre-defined variable names are:

     $_    default variable for built-in functions
     @_    array containing arguments passed to a subroutine

Perl also pre-defines many other variables, which are discussed in other sections of this tutorial. Two such examples are:

     @ARG  array containing command line values
     %ENV  environmental values passed to Perl by web server

Case Sensitivity
Perl is case-sensitive. The variables $dog and $DOG are two different variables.

Creating a Variable
Unlike some languages which require that you define a variable before using it, Perl allows you to define a variable by simply using it in a line of code.

     $a = 5      # $a is created by assigning it a value
     my $b = 2   # the tutorial on scope discusses 'my'

When a variable has not yet been assigned a value, Perl sets the value to undef, which is similar to NULL in other languages. The Perl defined function can test to see if a variable has been assigned a value.

NameSpace
Note that $x, @x, and %x are different variables. They can coexist. These lines of code will create all three variables, which Perl maintains separately.

     $x = 2                   # scalar
     @x = (1,2)               # array
     %x = ("a",1,"b",2)       # hash 

Good Practice
Using variable names which describe the content of the variable is considered good practice. For example, $size is preferred over $s.

Programmer's also use the following two common variable naming practices, both of which involve using more than one word in the variable name.

     last_index     # words separated by an underscore
     LastIndex      # capitalizing words in the variable name