package C1;
sub foo { 
   my $self= shift;
   print "C1::foo called @_ :", join (',', keys %$self), "\n"; 
   }

package C2;
sub foo { 
   my $self= shift;
   print "C2::foo called @_ :", join (',', keys %$self), "\n"; 
   }

package main;
my $x= bless \%hash, "C1";
my $y= bless \%hash, "C2";
$x->foo ("before");
$y->foo ("before");
%hash= ( key=>'value');
$x->foo ("after");
$y->foo ("after");