Yes, you are reading this a bit wrong, but that's not uncommon - I struggled with this for some time too before I got it.

The @_ passed along will be the @_ at the time of call to the callback. Let's look at this in a simpler setting:

use strict;
use Data::Dumper;

sub parse {
    my ($self, $xmlfile) = @_;

    my $handler = sub { $self->handle_start(@_) };
    # time passes
    $handler->('foo','bar');
}

sub handle_start {
    print Dumper \@_;
};

sub new { bless {} };

my $self = main::->new();
$self->parse('some.xml');
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This outputs

$VAR1 = [
          bless( {}, 'main' ),
          'foo',
          'bar'
        ];
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As you see, the arguments to $handler are foo and bar, and the callback adds $self in front of those. The @_ in the anonymous subroutine does refer to the @_ at the moment of the call to $handler (in fact, there only ever is one @_, and it is always the one for the current call to the current subroutine). I hope that clears it up somewhat.