use Data::Dump qw(pp);
my @array1=(1..5);
my @array2=(3..7);

my (%union,%intersection,%difference,@tmp);

my (%set1,%set2);
@set1{@array1}=@array1;
@set2{@array2}=@array2;


@tmp= @set1{ keys %set2 };
@intersection{@tmp}=@tmp;     # warning: use of uninitialized values
delete $intersection{""} ;    # dirty hack



%union=(%set1,%set2);


%difference=%union;
delete @difference{keys %intersection};


pp \(%set1,%set2,%union,%intersection,%difference);

##</code><code>##

Use of uninitialized value $tmp[0] in hash slice at /home/lanx/B/PL/PM/set.pl line 17.
Use of uninitialized value $tmp[3] in hash slice at /home/lanx/B/PL/PM/set.pl line 17.
(
  { 1 => 1, 2 => 2, 3 => 3, 4 => 4, 5 => 5 },
  { 3 => 3, 4 => 4, 5 => 5, 6 => 6, 7 => 7 },
  { 1 => 1, 2 => 2, 3 => 3, 4 => 4, 5 => 5, 6 => 6, 7 => 7 },
  { 3 => 3, 4 => 4, 5 => 5 },
  { 1 => 1, 2 => 2, 6 => 6, 7 => 7 },
)