package Foo;

sub new {
    my $class = shift;

    return bless {}, $class;
}

package Foo::Bar;

use base 'Foo';

package main;

use Foo::Bar;

my $foobar = Foo::Bar->new;
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Because there isn't a new() method in the Foo::Bar class, it will use the new() method in its parent class, which is Foo. Because Foo's new() method uses the $class that was passed into new() (which is Foo::Bar), it will DWIM.