package My::Parent;
our %ClassData;
my $ClassCount;
BEGIN { %ClassData= ( this=>"that",
    foo=>"baz", count=>\$ClassCount ) }
sub new {
    my $this= shift;
    my $class= ref($this) || $this;
    my $classData;
    {   no strict 'refs';
        $classData= \%{$this."::ClassData"};
    }
    #...
    $self{foo}= $classData->{foo};
    ++${$classData->{count}};
    return $self;
}
package My::Son;
use base "My::Parent";
*{ClassData}= \%My::Parent::ClassData;
package My::Daughter;
use base "My::Parent";
our %ClassData;
my $DaughterCount;
BEGIN {
    %ClassData= %My::Parent::ClassData;
    $ClassData{foo}= "bar";
    $ClassData{count}= \( $DaughterCount= 0 );
}