Damian's example is fine if you need to hide all variables behind methods, but the use of closures often confuses newbies. The simple answer to this question is to just use a (package-scoped) global:

use strict;

my @foo;
push @foo, Foo->new() for (0..4);

package Foo;
use vars qw($count);

sub new { 
   my $class = shift;

   $count++;
   print STDERR 'there are ', $count, " instances\n";

   my $self  = {
      bar => 42
   };
   return bless $self, $class;
}
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