${ X() } . ${ X() }

   concat(
      deref( X() ),
      deref( X() ),
   )


${ X() } . ' ' . ${ X() }

   concat( 
      concat(
         deref( X() ),
         ' ',
      ),
      deref( X() ),
   )

##</code><code>##

use strict;
use warnings;
use feature qw( say );

{my$x=0; sub X{++$x;\$x}}

sub concat { $_[0] . $_[1] }
sub deref :lvalue { ${ $_[0] } }

# 22
say
   concat(
      deref( X() ),
      deref( X() ),
   );

# 3 4
say
   concat( 
      concat(
         deref( X() ),
         ' ',
      ),
      deref( X() ),
   );