use strict;
use warnings;

package UI;
our @ISA = ();
sub new { my $class = shift; return bless {@_}, $class }
sub hello {print "superclass hello\n"};

package UI::Graphical;
our @ISA = 'UI';
sub hello {print "make a window\n";}

package UI::Text;
our @ISA = 'UI';
sub hello {print "hello world!\n";}

my $ui = UI->new;
$ui->hello;

my $tui = UI::Text->new;
$tui->hello;

my $gui = UI::Graphical->new;
$gui->hello;