# List pair which has one old item to be deleted,
# and one new item to be added:
@oldlist = (  1,3,5,7,9     );
@newlist = (    3,5,7,9,11  );

foreach $item(@oldlist){
  $old{$item}=1;
}
foreach $item(@newlist){
  $new{$item}=1;
}

# Find the stuff that's on both lists:
my @bothlists = grep {defined $new{$_}} @oldlist;
print "Stuff on both lists: @bothlists\n";

# Find the stuff that's on the old but not the new list:
my @deletions = grep {!defined $new{$_}} @oldlist;
print "Stuff on old but not on new (needs deleting): @deletions\n";

# Find the stuff that's on the new but not the old list:
my @additions = grep {!defined $old{$_}} @newlist;
print "Stuff on new but not on old (needs adding) @additions\n";