#!/usr/bin/perl
use warnings;
use strict;
use Data::Dumper;

my @h = (
    { A => 1, B => undef },
    { A => 2, B => 2 },
    { A => 2, B => undef },

);

$_->{B} ||= $_{A} foreach @h;  # Bug!
print Dumper(\@h);