package main;

use strict;
use warnings;
use CGI;

package CGI;

sub foo {
    my $self = shift;
    return $self->p('foo');
} 

package main;

my $cgi = CGI->new;
print $cgi->foo() . "\n";
[download]

Thank you for this great example.

Since you have direct access to the package via its namespace, you don't even need the package switching. This works too–

use CGI;

my $cgi = CGI->new;
print $cgi->foo(), "\n"; 

sub CGI::foo {
    my $self = shift;
    return $self->p('foo');
}
[download]

Unless you go to extraordinary length to close them, most implementations, including the defacto blessed-reference based and inside-out implementations of classes allow you to add or replace methods at runtime.

For my interest, what is it that you find so desirable about open classes?

I think I like them because you don't have to spend all that time on design patterns.

Sorry, but you'd have to expand that a little before I'll understand your reasonimg?

---

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"Science is about questioning the status quo. Questioning authority".

In the absence of evidence, opinion is indistinguishable from prejudice.

"Too many [] have been sedated by an oppressive environment of political correctness and risk aversion."