package MessageLibrary;
use ...

my $hash = { ... };

sub new {
  my ( $class, %args ) = @_;
  my $self = {}; bless ( $self, $class );
  $self->_init();
  return $self;
}

sub _init { ... }
sub AUTLOAD() { ... }

1;

#-----------------------------

package MessageLibrary::Error;

my $hash = { ... something different ... }

sub _init {
  my ( $self, %args ) = @_;
  ... class-specific init code, if needed ...
  $self->SUPER::_init ( %args );
}

1;