foo.pm:
package foo;
use Hash::Util;
use strict;
use warnings;

sub new {
    my $self = {};
    $self->{XBCDE} = 1;
    $self->{AXCDE} = 2;
    $self->{ABXDE} = 3;
    $self->{ABCXE} = 4;
    $self->{ABCDX} = 5;
    bless($self);
    Hash::Util::lock_keys(%{$self});
    return $self;
}

sub bar_1 {
    my $self = shift;
    $self->{AXCDE} = 10;
}

sub bar_2 {
    my $self = shift;
    print $self->{AXCDE};
}

sub bar_3 {
    my $self = shift;
    $self->{XXXXX} = 10;
}

sub bar_4 {
    my $self = shift;
    print $self->{XXXXX};
}

1;

foo.pl:
use foo;
use strict;
use warnings;

my $foo = new foo;
$foo->bar_1; #okay as the key exists
$foo->bar_2; #okay as the key exists
$foo->bar_3; #no good, comment out this, and try for the second time
$foo->bar_4; #no good, comment out this, and try for the third time