my %hash = (name => 'andrew', beer => 'Ale');
foreach my $KV (%hash) {
    print ">>$KV<<\n";
    $hash{$KV} = 'Dark Ale' if $KV eq 'beer';
}
__END__
# with 5.00503:
>>name<<
>>andrew<<
>>beer<<
>>Ale<<

# with 5.6.1
>>beer<<
>>Dark Ale<<
>>name<<
>>andrew<<