One possible usage: imagine if you had some sort of factory method and wanted to create subclasses on the fly. Handwavy terrible example just what you could do with it (use a real metaprogramming module instead):

## In Widget
sub make_color_widget {
  my $class = shift;
  my $new_color = shift;

  my( @widget_args ) = @_;


  { no strict refs;
    @{ qq{Widget::${new_color}::ISA } = (qw( Widget ) );
  } 

  my $self = {@widget_args};

  return bless $self, qq{Widget::$new_color};
}

sub my_color {
  my $self = shift;
  return ( (ref $self) =~ m{^Widget::(.*)} )[0];
}
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Then you could do something like:

$ perl -I . -MWidget -de 0

Loading DB routines from perl5db.pl version 1.57
Editor support available.

Enter h or 'h h' for help, or 'man perldebug' for more help.

main::(-e:1):   0
  DB<1> $red = Widget->make_color_widget( red => () );

  DB<2> $blue = Widget->make_color_widget( blue => () );

  DB<3> x $red->my_color
0  'red'
  DB<4> x $blue->my_color
0  'blue'
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Rather than cheating and scraping from the instance's class name, you could use that as a key to lookup whatever in a hash keyed by the class name. Or you could (dynamically) generate code and populate subs in the new class' package namespace (*{"Widget::${new_color}::frobulate} = sub { ... }

Edit: a possible usage other than the obvious that's how subclasing works, of course. (derp)