my $fields = join "||'|'||", qw( field1, field2, field3);
my %set1 = map { $_ => 1 }
  get_records ( "SELECT $fields FROM table WHERE data_set = 'set1'");
my %set2 = map { $_ => 1 }
  get_records ( "SELECT $fields FROM table WHERE data_set = 'set2'");

my @set2not1 = grep { ! exists($set1{$_}) } keys %set2;
my @set1not2 = grep { ! exists($set2{$_}) } keys %set1;