Theres a section in Mastering Algorithms with Perl on sets also. If you need a full implementation of unions/differences/intersections etc see the Set::Scalar module. If you use chromatic's code linked to above, I gave into my compulsion to do a tiny bit of benchmarking:

my @kansas = qw( corn wheat hay cattle );
my @oklahoma = qw( wheat dairy cattle );

use Benchmark;
timethese(100000, {
  "diff1" => sub { diff1(\@kansas, \@oklahoma) },
  "diff2" => sub { diff2(\@kansas, \@oklahoma) },
});

sub diff1 {
  my %hash; 
  @hash{@{$_[1]}} = (1) x @{$_[1]};
  return grep { !defined $hash{$_} }  @{$_[0]};
}

sub diff2 {
  my %hash = map{ $_=>1} @{$_[1]}; 
  return grep { !defined $hash{$_} }  @{$_[0]};
}
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It seems that a hash slice is a teeny bit faster than map for this purpose:
Results:

Benchmark: timing 100000 iterations of diff1, diff2...
     diff1:  2 wallclock secs ( 1.74 usr +  0.01 sys =  1.75 CPU)
     diff2:  3 wallclock secs ( 2.22 usr +  0.01 sys =  2.23 CPU)
[download]