Basically, that construct is saying "This method can be called as either a class or instance method".

merlyn's writings have been against using this in constructors specifically, not against using it in general methods to allow it to be called against both a class and an object. This is because a chunk of programmers out there expect it to act as a clone, another chunk thinks it should act as a copy, and yet another chunk thinks it should be an error.

I go for something like this:

sub new 
{
    my $class = shift;
    die "Need to be called as a class method" 
        if ref $class;
    my %params = %{ +shift };

    # Define what params we expect
    my %self = map { $_ => $params{$_} || '' } qw/
        foo bar baz
    /;

    # Validation

    bless \%self => $class;
}
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