#!/usr/bin/perl -w
use strict;

# this initialization to () is not necesssary, but ok
my (@union, @intersection, @difference, %count) = ();

my @array1 = (1,2,3);
my @array2 = (3,4,1);

foreach my $element (@array1, @array2) 
{
    $count{$element}++; #auto-vivification of a hash entry
}

foreach my $element (keys %count)  # a print to show results so far
{
   print "$element occured $count{$element} times\n";
}

@union = keys %count; # same as the push @union, $element statement
                      # it would be faster to have this within the
                      # foreach loop, because calculating the keys
                      # takes some "work", but I'm showing here 
                      # what this does. @union and keys %count are
                      # equivalent.

foreach my $element (keys %count)
{
   if ($count{$element} > 1)  #element occured more than once
   {
      push @intersection, $element;
   }
   else                       #element only occured once
   {
      push @difference, $element;
   }
}

print "union=@union; intersection=@intersection; difference=@difference\n";

__END__
4 occured 1 times
1 occured 2 times
3 occured 2 times
2 occured 1 times
union=4 1 3 2; intersection=1 3; difference=4 2