#!/usr/bin/perl -w

use strict;

my @red_things = qw(apple little_corvette strawberry lava);
my @fruits = qw(apple pineapple strawberry banana);

my @sorted_red_things = sort @red_things;
my @sorted_fruits = sort @fruits;

while (@sorted_red_things || @sorted_fruits)
{
  my($thing,$fruit)=($sorted_red_things[0],$sorted_fruits[0]);
  if (defined($thing) and (!defined($fruit) or ($thing lt $fruit)))
  {
    print "A red thing that's not a fruit: $thing\n";
    shift @sorted_red_things;
  }
  elsif (!defined($thing) or ($thing gt $fruit))
  {
    print "A fruit that's not red: $fruit\n";
    shift @sorted_fruits;
  }
  elsif ($thing eq $fruit)
  {
    # It's in both lists
    shift @sorted_red_things;
    shift @sorted_fruits;
  }
}