@union = @intersection = @difference = ();
%count = ();
foreach $element (@array1, @array2) { $count{$element}++ }

foreach $element (keys %count) {
  push @union, $element;
  push @{ $count{$element} > 1 ? \@intersection : \@difference
  }, $element;
}

##</code><code>##

use strict;

my (@local) = get_local_ids();
my (@remote) = get_remote_ids();

my %have_local = ();
foreach $element (@local) { $have_local{$element}++ };

foreach $id (@remote) {
  next if $have_local{$id};
  retrieve($id);
  $have_local{$id}++;
}