Programming Perl, 4th Edition

Package-Quoted Classes

The final syntactic ambiguity with the indirect object style of method invocation is that it may not be parsed as a method call at all, because the current package may have a subroutine of the same name as the method. When using a class method with a literal package name as the invocant, there is a way to resolve this ambiguity while still keeping the indirect object syntax: package-quote the classname by appending a double colon to it.

$obj = method CLASS::;   # forced to be "CLASS"–>method

This is important because the commonly seen notation:

$obj = new CLASS;        # might not parse as method

will not always behave properly if the current package has a subroutine named new or CLASS. Even if you studiously use the arrow form instead of the indirect object form to invoke methods, this can, on rare occasion, still be a problem. At the cost of extra punctuation noise, the CLASS:: notation guarantees how Perl will parse your method invocation. The first two examples below do not always parse the same way, but the second two do:

$obj = new ElvenRing;               # could be new("ElvenRing")
                                    # or even new(ElvenRing())
$obj = ElvenRing–>new;              # could be ElvenRing()–>new()

$obj = new ElvenRing::;             # always "ElvenRing"–>new()
$obj = ElvenRing::–>new;            # always "ElvenRing"–>new()

This package-quoting notation can be made prettier with some creative alignment:

$obj = new ElvenRing::
            name    => "Narya",
            owner   => "Gandalf",
            domain  => "fire",
            stone   => "ruby";

Still, you may say, “Oh, ugh!” at that double ...