"As you see, I didn't know how to create and use what we say member variable in c++. The $XMLfile_name and the @hostList here is using as member variables. @hostList used for holding results parsed from the XML file."

As per $singleton = bless {}, $class, the singleton object is a blessed hash, right? Hmm... now I wonder how you can associate a couple bits of data with the object?

$singleton->{xml_filename} = $_[0];
$singleton->{hostlist}     = \@hostlist;
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"Another question: In the new method, as I read package method always put the invoker as the first argument. I suppose 'invoker' for new() means the class MyConfig and return an instance after blessed; And the 'invoker' for loadConfig seems change to be an instance, seems the $self by te first default argument is an instance not the class any more. But as I know the 'new' is an ordinary method as others, not a keywords, why they behaves different?"

The invocant is whatever is on the left side of the arrow operator ->. So:

Dancer->new;     # invocant is "Dancer"
$obj->dance;     # invocant is $obj
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"The last question:I don't think I made my while condition correct. How can I deal with the while index simply using $_? Thanks a lot in advance!!"

You could use a foreach loop, a la:

my @components = @{ $config->{host}->{component} };
foreach (@components)
{
   push @hostList, $_->{component_name};
}
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Though personally I'd just use map; no explicit looping construct necessary:

push @hostList,
   map { $_->{component_name} }
   @{ $config->{host}->{component} };
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That said, I'd recommend looking at Moose and MooseX::Singleton. Moose takes away a lot of the boilerplate OO "plumbing" necessary in Perl, allowing you to concentrate on the interesting stuff.

{
    package MyConfig;
    use Moose;
    use MooseX::Singleton;
    use XML::Simple;

    has xml_filename => (
        is         => 'ro',
        isa        => 'Str',
        required   => 1,
    );

    has _config => (
        is         => 'ro',
        isa        => 'Any',
        lazy_build => 1,
    );

    has hostlist => (
        is         => 'ro',
        isa        => 'ArrayRef',
        lazy_build => 1,
    );

    sub _build__config {
        my $self = shift;
        my $xml  = XML::Simple->new;
        $xml->XMLin($self->xml_filename, ForceArray => [ 'host', 'comp
+onent' ])
            or die $!;
    }

    sub _build_hostlist {
        my $self = shift;
        my @hostlist = 
            map { $_->{component_name} }
            @{ $self->_config->{host}[0]{component} };
        return \@hostlist;
    }
}

my $config   = MyConfig->new(xml_filename => "myconfig.xml");
my @hostlist = @{ $config->hostlist };

print "@hostlist";
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