$ui->refresh

In the Text package, refresh is just a stub, i.e. refresh {}, while the Graphical package has a functioning refresh routine. (In general, the stubs might better be in the base class.) I create the object in the first lines of the program based on a command-line switch.

The program is a single file. I share data by using our ($var1, @var2, %var3) enabling all packages to access these variables without a package prefix.

This is how I got the polymorphism to work, thanks to the advice of some helpful monks.

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;
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UPDATE: Changed @ISA = '' to </c> @ISA = ()</c> as cautioned by chromatic.