Programming Perl, 4th Edition

New Tricks

As of v5.6, you can also declare a method to indicate that it returns an lvalue. This is done with the lvalue subroutine attribute (not to be confused with object attributes). This experimental feature allows you to treat the method as something that would appear on the lefthand side of an equals sign:

package Critter;

sub new {
    my $class = shift;
    my $self = { pups => 0, @_ };    # Override default.
    bless $self, $class;
}

sub pups : lvalue {                  # We'll assign to pups() later.
    my $self = shift;
    $self–>{pups};
}

package main;
$varmint = Critter–>new(pups => 4);
$varmint–>pups *= 2;                 # Assign to $varmint–>pups!
$varmint–>pups =~ s/(.)/$1$1/;       # Modify $varmint–>pups in place!
print $varmint–>pups;                # Now we have 88 pups.

This lets you pretend $varmint–>pups is a variable while still obeying encapsulation. See the section The lvalue Attribute in Chapter 7.

If you’re running a threaded version of Perl and want to ensure that only one thread can call a particular method on an object, you can use the locked and method attributes to do that:

sub pups : locked method {
    ...
}

When any thread invokes the pups method on an object, Perl locks the object before execution, preventing other threads from doing the same. See the section The method Attribute in Chapter 7.