In perl, it makes no sense at all.

Operand evaluation order is actually well defined and system-independent even though it's not documented for most operator. Operands are evaluated as late as possible, left-to-right except for assignment operators which are right-to-left. (Exponentiation's operand evaluation order is left-to-right, so its operand evaluation order differs from its operator associativity.) 

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

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


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

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

[download]

The key here is that ${ X() } returns $x itself, not a copy of it. 

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() ),
   );

[download]

In reply to Re^5: Order of evaluation/interpolation of references (op order) by ikegami
in thread Order of evaluation/interpolation of references by Anonymous Monk

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