ex10.pm:

package ex10;

use Hash::Util;

sub new {
    my $self = {};
    $self->{ATTR_ABXCDEFG} = 1;
    $self->{ATTR_ABCXDEFG} = 2;
    $self->{ATTR_ABCDXEFG} = 3;
    $self->{ATTR_ABCDEXFG} = 4;
    $self->{ATTR_ABCDEFXG} = 5;
    bless($self);
    Hash::Util::lock_keys(%{$self});
    return $self;
}

sub do_something_1 {
    my $self = shift;
    $self->{ATTR_ABCXDEFG} = 10;
}

sub do_something_2 {
    my $self = shift;
    print $self->{ATTR_ABCXDEFG};
}

sub do_something_3 {
    my $self = shift;
    $self->{ATTR_AXBCDEFG} = 10;
}

sub do_something_4 {
    my $self = shift;
    print $self->{ATTR_AXBCDEFG};
}

1;

ex10.pl:

use ex10;

$e = new ex10;
$e->do_something_1;
$e->do_something_2;
$e->do_something_3; #see what will happen, if you comment out this
$e->do_something_4; #try to comment out this