@ISA=qw(Sclass, Wclass);
....
new(){
my $s=Sclass->new();
TELL_ISA_LOOKUP_USING(Sclass, 'self') #(self=this class's handle)
# and use the "wclass_handle_field as handle for Wclass:
TELL_ISA_LOOKUP_USING(Wclass, 'self'->{'wclass_handle_field'})
....
}