package Foo;
my @_instances;
sub new {
  ...
  push @_instances, $self;
  return $self;
}

sub zorbnitz { print "Weeble\n"; }

sub for_All_Foos {
  my $class = shift;
  my $code  = shift;

  $code->( $_ ) for @_instances;
}

package main;

$a = Foo->new;
$b = Foo->new;

Foo->for_All_Foos( sub { $_[0]->zorbnitz } );