use strict;

sub dec($) {
  $_[0] ? $_[0]--
        : 0
};

my @a1 = qw(1 2 2 3);
my @a2 = qw(1 2 3 3);

my %a1;

$a1{$_}++
  for (@a1);

my @only_a2 = map { dec $a1{$_} ? () : $_ } @a2;
print "Only in \@a2 : @only_a2\n";