my $plugins = {

        foo => sub { our $version = '0.001';
                    ...do some tom-foo-lery...

                  },

   foo_bar => sub { 

      our $version = '0.001';

      if (defined $plugins->{'foo'})
        { 
         if ($plugins->{'foo'}->version ge '0.001')
           {

            ... do some foo-bar stuff ...
     
             my $var = $plugins->{'foo'}->("hello");


            ... do some more foo-bar stuff ...

            return "result of foo-bar";
           }
          
         return 'foo_bar error; please update foo';
        }

      return 'foo_bar error; please install foo';
     }
 
   };