G'day mr.nick,

It looks like you've resolved your posted question with ++choroba's help.

As you "do very, very little OOP in Perl", I thought I'd draw your attention to the parent pragma which is used to "Establish an ISA relationship with base classes at compile time".

Using this pragma, your code might look at little more straightforward written along these lines:

$ perl -Mstrict -Mwarnings -E '
    package Event;
    sub new {
        my $class = shift;
        my $self = { EventType => "Windows 2003", @_ };
        bless $self => ref $class || $class;
    }

    package Event08;
    use parent qw{-norequire Event};
    sub new {
        shift->SUPER::new(EventType => "Windows 2008", @_);
    }

    package Security;
    use parent qw{-norequire Event};
    sub new {
        shift->SUPER::new(EventLog => "Security", @_);
    }

    package Security08;
    use parent qw{-norequire Event08};
    sub new {
        shift->SUPER::new(EventLog => "Security", @_);
    }

    package main;
    my $sec = Security::->new(some_attribute => "abc");
    say "Class:          ", ref $sec;
    say "EventType:      ", $sec->{EventType};
    say "EventLog:       ", $sec->{EventLog};
    say "Some attribute: ", $sec->{some_attribute};

    my $sec08 = Security08::->new(some_attribute => "xyz");
    say "Class:          ", ref $sec08;
    say "EventType:      ", $sec08->{EventType};
    say "EventLog:       ", $sec08->{EventLog};
    say "Some attribute: ", $sec08->{some_attribute};
'
Class:          Security
EventType:      Windows 2003
EventLog:       Security
Some attribute: abc
Class:          Security08
EventType:      Windows 2008
EventLog:       Security
Some attribute: xyz
[download]

Notes:

• It's considered good practice to put classes in their own <ClassName>.pm files. If you do this, you won't need the -norequire: see the parent documentation for details.
• What I've shown under package main; would typically be in one or more <script_name>.pl files. If you do this, you won't need package main;. However, you would need use Security; and use Security08; (with the test code I've shown).