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

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

##</code><code>##

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

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

##</code><code>##

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

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

##</code><code>##

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

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

##</code><code>##

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

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

my $a = \ '1';
my $b = \ '2';

say concat( deref( $a ), deref( $b ), );
say $$a;
say <<'EOS';
Note: $$a has not changed, So, $$a was copied to produce the concatenation
1 + 1 = 2 bytes copied.
EOS

say concat( concat( deref( $a ), ' ', ), deref( $b ), );
say <<'EOS';
Note: $$a has not changed, So, $$a was copied to produce the concatenation
Therefore, the whole of that result was copied (again)
to produce the result of the second concatenation.
( 1 + 1 ) * 2 + 1 = 5 bytes copied.


EOS
}

package buk; {
sub concat { $_[0] .= $_[1] }
sub deref  { ${ $_[0] } }
sub new{ "$_[0]" }

my $a = \ '1';
my $b = \ '2';

say concat( concat( new( deref( $a ) ), ' ', ), deref( $b ), );
say <<'EOS';
New copies 1 byte its arg into a new string
The first concat() copes the space and apend it to the new string.
The second concat() copies 1 byte from $b and apend it to that.
1 + 1 + 1 = 3 bytes copied.
EOS
}

__END__
C:\test>junk999
12
1
Note: $$a has not changed, So, $$a was copied to produce the concatenation
1 + 1 = 2 bytes copied.

1 2
Note: $$a has not changed, So, $$a was copied to produce the concatenation
Therefore, the whole of that result was copied (again)
to produce the result of the second concatenation.
( 1 + 1 ) * 2 + 1 = 5 bytes copied.



1 2
New copies 1 byte its arg into a new string
The first concat() copes the space and apend it to the new string.
The second concat() copies 1 byte from $b and apend it to that.
1 + 1 + 1 = 3 bytes copied.