# common, clean, readable, OO-ish way
 $foo->bar ( 'bash' );

 # less common, probably less readable, way
 Foo::bar ( $foo, 'bash' );

##</code><code>##

# from 5.6.1's Socket.pm on Linux 2.4
sub AUTOLOAD {
    my($constname);
    ($constname = $AUTOLOAD) =~ s/.*:://;
    my $val = constant($constname, @_ ? $_[0] : 0);
    if ($! != 0) {
        my ($pack,$file,$line) = caller;
        croak "Your vendor has not defined Socket macro $constname, used";
    }
    eval "sub $AUTOLOAD () { $val }";
    goto &$AUTOLOAD;
}

##</code><code>##

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;