If you want to see what's happening, you can step through the following: 
use strict;
use warnings;

use Data::Alias qw( alias );

my @stack;

# Increments the scalar on the stack.
sub preinc {
   alias my $i = pop(@stack);
   ++$i;
   alias push @stack, $i;
}

# Increments the scalar on the stack.
# Returns a new scalar with the original
# value of the scalar on the stack.
sub postinc {
   alias my $i = pop(@stack);
   my $j = $i;
   alias push @stack, $j;
   ++$i;
}

sub add {
   alias my $j = pop(@stack);
   alias my $i = pop(@stack);
   my $sum = $i + $j;
   alias push @stack, $sum;
}


{
   my $x=5;

   # $x + ++$x + $x++
   alias push @stack, $x;
   alias push @stack, $x;
   preinc;
   add;
   alias push @stack, $x;
   postinc;
   add;

   print "$stack[-1]\n";   # 18
}

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(alias $x = $y; represents the copying of a C pointer rather than copying the actual scalar.)

In reply to Re: question 1st - undefined behaviour by ikegami
in thread question 1st - undefined behaviour by rumos2

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