6.1 Basic Package

The package keyword signifies the beginning of a new namespace. All global identifiers (names of variables, subroutines, filehandles, formats, and directory handles) mentioned after this statement "belong" to that package. For example:

package BankAccount;
$total = 0;
sub deposit {
    my ($amount)= @_;
    $total += $amount;
    print "You now have $total dollars \n";
}
sub withdraw {
    my ($amount)= @_;
    $total -= $amount; 
    $total = 0  if $total < 0;
    print "You now have $total dollars \n";
}

The user-defined global identifiers $total, deposit, and withdraw belong to the BankAccount package. The scope of a package lasts until the end of the innermost enclosing block (if it is declared inside that block) or until another package statement is encountered. In the absence of an explicit package declaration, Perl assumes a package name called main.

This is how you use the global symbols from another package:

package ATM;               # Start a different name-space now
BankAccount::deposit(10);  # Call a foreign subroutine
print $BankAccount::total; # Access a foreign variable

To access an identifier in a different namespace, you need to put the package name before the variable name; this is called fully qualifying the name. Note that you must say $BankAccount::total, not BankAccount::$total; the $ sign is followed by the fully qualified name. If an identifier is not fully qualified, Perl looks for it in the currently active package.

Since the package statement simply dictates the effective namespace, you can switch between different namespaces at will:

package A;
$a = 10;      # This $a is in package A
package B;
$a = 20;      # This $a is in package B, and is completely independent
              # of the other $a
package A;     # Make A the current package.
print $a;     # prints 10;

C++ programmers will recognize the resemblance to that language's namespace facility.

my $BankAccount::total; # Error

$x = 10 ;   # global to package main
my $x = 20; # lexical at file scope
print $x;   # prints 20. Lexical variables are given priority.

package A;
$x = 10;

package B;
# Access $A::x symbolically
print ${"A::x"};

# or even more indirectly
$pkg      = "A";
$var_name = "x";
print ${"${pkg}::$var_name"};

# Call a subroutine indirectly
&{"A::foo"}(10, 20); # Identical to A::foo(10,20);