sub Populate {
    my ( $self, $args ) = @_;
#-------------------------------------------------------------------

#take care of args which don't belong to the SUPER, see Tk::Derived

my $xtra_arg = delete $args->{-dooda};  #delete and read same time 
if( defined $xtra_arg ) { $self->{'dooda'} = $xtra_arg }

#-----------------------------------------------------------------
    $self->SUPER::Populate($args);
}

Otherwise, show a complete running example.