Values associated with scalar variables in Perl are either numbers or
character strings. Numbers correspond to double precision floating
point numbers. Variable names begin with the special character
$. For instance, we could have the following assignments in a
Perl program.
$num1 = 7.3; $str1 = "Hello123"; $str2 = "456World";
Variables (and their types) do not have to be declared. Moreover, the
type of a variable changes dynamically, according to the type of
expression that is used to assign it a value. We use normal
arithmetic operators to combine numeric values. The symbol .
denotes string concatenation. If a numeric value is used in a string
expression, it is converted automatically to the string representation
of the number. Conversely, if a string is used in a numeric
expression, it is converted to a number based on the contents of the
string. If the string begins with a number, the resulting value is
that number. Otherwise, the value is 0. Continuing our example
above, we have the following (text after # is treated as a
comment in Perl).
$num3 = $num1 + $str1; # $num3 is now 7.3 + 0 = 7.3 $num4 = $num1 + $str2; # $num4 is now 7.3 + 456 = 463.3 $str2 = $num4 . $str1; # $str2 is now "463.3Hello123" $str1 = $num3 + $str2; # $str1 is now the number 470.6 $num1 = $num3 . "*" . $num4; # $num1 is now the string "7.3*463.3"
By default, Perl values are made available to the program the moment they are introduced and have global scope and are hence visible throughout the program. Uninitialized variables evaluate to 0 or "" depending on whether they are used as numbers or strings. The scope of a variable can be restricted to the block in which it is defined by using the word my when it is first used.
my $a = "foo";
if ($some_condition) {
my $b = "bar";
print $a; # prints "foo"
print $b; # prints "bar"
}
print $a; # prints "foo"
print $b; # prints nothing; $b has fallen out of scope