I would have expected that an instance of the base class would have been returned.

You should read more on Object Oriented Programming. This is not how inheritance works. Try this:

{ package MyBase;
  sub new {
      my $class = shift;
      warn "Constructing $class\n";
      bless {}, $class;
  }
}

{ package MyChild;
  use base 'MyBase';
}

package main;
use feature 'say';
my $p =  MyBase->new;
my $ch = MyChild->new;
say for $p, $ch;
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Updated: MyBase quoted in "use base".