Is it bad form to come back and answer my question? I looked at "comparing two arrays" in categorised Q and A, and dumbed it down a little to this: 
# 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";

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In reply to Re: comparing two hashes by Cody Pendant
in thread comparing two hashes by Cody Pendant

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